Serotonin transporter binding with [¹²³I]β-CIT SPECT in major depressive disorder versus controls: effect of season and gender

After receiving approval of the institutional ethics committee and written informed consent, we recruited depressed patients from primary care, and our outpatient department (October 2003 to August 2006). Patients were eligible if they were 25–55 years old, had a diagnosis of MDD (diagnosed by structured clinical interview for DSM-IV, SCID patient version), had a HDRS score of >18, were antidepressant-free, and were using no more than one antidepressant (stopped for >4 weeks and ≥5 half-lives of this antidepressant before scanning) for the present MDD episode. Exclusion criteria were pregnancy (or desire to become pregnant), bipolar disorder, psychotic features, primary anxiety and/or substance abuse disorders, and acute, severe suicidal ideation. Secondary comorbid anxiety and/or substance abuse were allowed.

We individually matched each patient by gender and age (±2 years). Healthy controls were in good physical health and had never used psychotropic medication. Exclusion criteria were current or life-time psychiatric disorder(s) according to the SCID (including abuse or addiction disorders), a Beck depression inventory (BDI) score of >9, alcohol use >4 units per day (last month) or a first-degree relative with psychiatric disorder(s). We allowed the controls to have incidentally used illicit drugs unless criteria for a DSM-IV disorder were met, but prohibited illicit drug use the month prior to scanning. Illicit drug use was not tested at the time of scanning. Patients and controls received €50 and €40, respectively. No restrictions were made with respect to smoking behaviour.

We performed all scans 230 ± 18 min (mean±SD) after intravenous injection of approximately 100 MBq [¹²³I]β-CIT, when the radioligand was at equilibrium for SERT binding in brain areas expressing high densities of SERTs [22]. Radiosynthesis of [¹²³I]β-CIT and image acquisition were as described previously [26]. We performed SPECT imaging using a 12-detector single slice brain-dedicated scanner (Neurofocus 810; Strichmann Medical Equipment, Cleveland, OH) with a full-width at half-maximum resolution of 6.5 mm, throughout the 20-cm field-of-view. The Neurofocus system is an upgrade of the SME 810 system [27], and acquires sequential single transaxial brain sections. Up to 24 axial sections 5 mm apart were scanned, and the energy window (140–178 keV) was placed symmetrically around the ¹²³I gamma energy of 159 keV. This system uses 800-hole long-bore, point-focused, collimators to obtain high-resolution images.

After attenuation correction (based on an automatically detected ellipse matching the outer head surface), and reconstruction in 3-D mode (based on a maximum a-posteriori reconstruction), we selected regions of interest (ROIs) for the midbrain, diencephalon and cerebellum by using validated templates (Fig. 1) [26]. One examiner (H.G.R.), blinded to the diagnosis, positioned all ROIs in two series. Intraclass correlation coefficients were ≥0.98 for all ROIs. If the two series differed by >5%, scans were reevaluated by a second investigator (J.B.). In the analyses the counts for the two series were averaged.

We assumed that activity in the cerebellum represented nondisplaceable activity (nonspecific binding and free radioactivity) [28]. We calculated the binding potential (BP) as the rate of specific to nondisplaceable (ND) binding (Z) for midbrain and diencephalon [29]. BP_ND is proportional to transporter number under equilibrium conditions.

General linear models were used to analyse differences in BP_ND in the midbrain and diencephalon between depressed patients and controls using the following modelling strategy.

We first compared mean BP_ND between MDD patients and controls in univariable (‘crude’) models, only containing the main effect of diagnosis (categorical: MDD/control). We then fitted multivariable models by adding variables to the model, which in the literature have been reported to influence BP_ND. These variables included: gender (categorical: male/female), age (continuous), smoking (categorical: yes/no), season of scanning (categorical: “winter”/“summer”; winter October–March, summer April–September [24, 25]). In addition, a number of specific two-way interactions were examined, again because they have previously been reported to be significant, which included: diagnosis×gender, diagnosis×smoking, gender×smoking (‘full multivariable models’) [11, 21]. Three-way interactions were not examined because of the relatively small sample size (from a statistical perspective). The Akaike’s information criterion (AIC) was used to judge whether the two-way interactions improved the model. If a two-way interaction did not improve the fit of the model, it was removed from the model in order to facilitate the interpretation of the model (‘reduced multivariable models’). Main effects always remained in the model, irrespective of their significance, in order to report their (lack of) impact. Diencephalon and midbrain data were analysed separately, but the same set of variables and interaction terms were examined using the same modelling approach. If significant interactions were present, post-hoc analyses were performed in order to report the absolute differences in SERT availability in the involved subgroups. Differences between subgroups were analysed within the framework of the multivariable model and tested for significance using the residual variance estimate of the model.

We examined the association of BP_ND with HDRS scores using linear regression models in patients, correcting for covariates used in the multivariable models. We used SPSS (version 15.0.1) for statistical procedures (http://​www.​spss.​com). All results are expressed as means±SD, except in Fig. 2, where ±SEM was used for legibility.