M7. LOWER THALAMIC DOPAMINE D2-RECEPTOR BINDING IN DRUG-NAIVE PATIENTS WITH PSYCHOSIS – A REPLICATION STUDY USING POSITRON EMISSION TOMOGRAPHY
Authors: Victorsson Pauliina , Plavén-Sigray Pontus , Matheson Granville , Santillo Alexander , Lee Maria , Collste Karin , Fatouros-Bergman Helena , Agartz Ingrid , Halldin Christer , Farde Lars , Cervenka Simon 
Published: May 2020

Abstract

Background
The dopamine system is a central focus of research on the pathophysiology and treatment of schizophrenia. With regard to the dopamine D2-receptor (D2-R), Positron Emission Tomography (PET) studies have shown a small increase in striatal receptor availability. In contrast, a more recent line of research has demonstrated lower levels of D2-R (Cohen’s D = -0.32) in the thalamus, a region of key interest for the pathophysiology of schizophrenia. However, some studies included patients previously on antipsychotic medication, or were performed using radioligands with suboptimal affinity for the much lower D2-R density in thalamus compared to striatum. In addition, the resolution of previous PET systems has not allowed for a more detailed analysis of functional thalamic subregions. Here we examined a fully antipsychotic-naïve sample of first-episode psychosis patients using the high-affinity D2-R radioligand [11C]FLB457 and high-resolution PET. The aim was to a) replicate previous findings of lower D2-R in thalamus in patients and b) specifically examine patient-control differences in thalamic subregions based on their cortical connectivity.

Methods
Nineteen antipsychotic-naïve first episode psychosis patients (mean age = 29.3; sd = 6.3, 11 males) and 19 age- and sex matched healthy comparison subjects were included in the analysis. PET measurements were obtained using a High Resolution Research Tomograph (HRRT). A ROI for whole thalamus was defined using the FSL Harvard Oxford Subcortical Atlas, whereas ROIs for thalamic subregions were based on the Oxford Thalamic Connectivity Atlas. Binding potential (BPND) was calculated using the Logan graphical analysis with cerebellum as reference region. The statistical analyses, which were all pre-registered, were performed using frequentist and Bayesian paired-samples t-tests.

Results
The frequentist paired t-test showed that patients had significantly lower binding than control subjects in whole thalamus (Cohen’s D = -0.479, p = 0.026). Bayes factor from the Bayesian paired t-test indicated that there was approximately 5 times more support for the hypothesis of lower BPND in patients, compared to the null hypothesis of no difference. Among subregions, the ROI corresponding to prefrontal thalamic connectivity showed the largest effect (Cohen’s D = -0.527, p = 0.017), and Bayes factor indicated that there was 6 times more support for lower BPND in patients compared to no difference.

Discussion
Using high resolution PET and a high affinity D2-R radioligand in antipsychotic-naïve first-episode psychosis patients, this study replicates the previously reported meta-analytical effect size of lower thalamic receptor availability in patients. The strongest effect was observed in the subregion dominated by connections to prefrontal cortex. The findings may reflect a dysregulation of the thalamic dopamine system in schizophrenia, which in turn could underlie aberrant functional connectivity in key fronto-thalamic circuits.