**4. Discussion**

In this exploratory study, we found no significant differences in the striatal and extrastriatal [18F]-FDOPA uptake between unmedicated autistic adults and controls. In the ASD sample, but not in the control or combined samples, the AQ attention to detail subscale scores were significantly and negatively correlated with dopamine synthesis capacity in the whole striatum, the putamen, and particularly the nucleus accumbens, although these findings were not supported by Bayesian analyses.

The results of our exploratory analyses confirm our previous ROI analysis, in which we found no differences in [18F]-FDOPA uptake in the striatum and its functional sub-regions (i.e., associative, limbic, and sensorimotor striatum) between ASD and controls [9]. We extend these findings by showing that [18F]-FDOPA uptake does not differ in anatomical sub-regions of the striatum (i.e., putamen, nucleus accumbens, and caudate nucleus) nor in extrastriatal brain regions. In the striatum, [18F]-FDOPA is decarboxylated to fluorodopamine through amino acid decarboxylase (AADC) and stored in vesicles within presynaptic terminals [20]. Although it is well-established that striatal [18F]-FDOPA uptake represents dopamine synthesis capacity, the radiotracer is taken up and stored by all AADC-containing, monoaminergic neurons [21]. On the one hand, this can be considered a limitation of the method, since to some extent it remains unknown what [18F]-FDOPA uptake in extrastriatal regions reflects. On the other hand, since we observed no significant

difference in uptake in the whole brain, this can indicate that, for instance, also serotonergic functioning in the raphe nuclei is unaltered in ASD [22].

Our findings partially differ from the study by Ernst et al. [5], who reported a decreased [18F]-FDOPA uptake in the anterior medial prefrontal cortex in autistic children (*n* = 14), and from the study by Nieminen von Wendt et al. [6], who found an increased [18F]-FDOPA uptake in the striatal and frontal regions in adults with Asperger syndrome (*n* = 8). Future studies can assess whether the differences between samples in factors, such as age, ASD diagnosis, and symptom severity, can have contributed to these partially discrepant findings.

Although we found no group differences in [18F]-FDOPA uptake, we did find significant negative associations between AQ attention to detail subscale scores and dopamine synthesis capacity in striatal ROIs among ASD adults. These findings should be interpreted with caution since multiple tests were performed, and Bayesian analyses were inconsistent with these observations. Nevertheless, it is of interest that a recent study also showed that in individuals with ASD (*n* = 18), but not in controls (*n* = 20), striatal dopamine D1 receptor binding was negatively associated with the same AQ attention to detail subscale [23]. This finding can be accounted for by either increased endogenous dopamine or by the expression of fewer D1 receptors. This latter explanation seems more plausible, since the authors note that the assessment of D1 receptor binding is unlikely to be strongly influenced by the availability of endogenous dopamine. Our findings of no increased striatal dopamine synthesis capacity in ASD, and a recent report of a decreased striatal dopamine release in response to monetary reward in adults with ASD (*n* = 10) compared to controls (*n* = 12) [24], support this interpretation, as these sugges<sup>t</sup> that endogenous synaptic dopamine is not higher in ASD. Together, these findings can then be interpreted as indicating that a reduction in striatal dopamine signaling is associated with attentional processes relevant to ASD, which accords with previous theoretical and empirical work on the role of striatal dopamine in ASD [1–3,25].

If striatal dopamine is indeed related to attentional processing in ASD, and we emphasize that this finding requires replication in an independent cohort, then it can do so in different ways. For example, striatal dopamine can be involved in the direction of attention to salient information [25–27], which would fit with our finding that associations were strongest in the nucleus accumbens, a region known to play a role in these cognitive processes [28]. It is also possible that our findings reflect alterations secondary to the perturbations in other neurotransmitter systems. Future studies, combining molecular imaging methods with objective assessments of cognitive functioning would be useful in this respect. Of note, there has been a relative scarcity of molecular imaging studies in ASD [4], and future (preferably longitudinal) assessments of different aspects of the dopamine and other neurotransmitter systems would help elucidate the role of these systems in ASD. Such assessments can also increase our knowledge on the reasons why certain medications might (not) work in autistic individuals. Given their high prescription rates [29], this seems useful and necessary.

The strengths of the present study are its large sample size and the completion of additional analyses to ensure the robustness of our findings. Note that, as reported previously [9], the mean cerebellar standardized uptake values were comparable for ASD and controls and, therefore, possible differences in non-specific uptake of [18F]-FDOPA can be excluded. A first limitation of the study is the exclusion of participants who used medication or had been diagnosed with a low IQ or a psychotic disorder, as we do not know how our findings generalize to those populations. Second, autistic traits were assessed by self-report only. Third, since the study was exploratory, we did not conduct a priori sample size calculations for the present study purposes. Fourth, data were collected on two PET/CT systems. However, reconstruction parameters for the two scanners were harmonized using published guidelines [30], and scanner type was added as a covariate to the analyses. Fifth, we chose to use [18F]-FDOPA analyses methods based on previous literature (e.g., [18]) and conducted additional sensitivity analyses (e.g., with varying *k*icer

thresholds); however, future studies should explore the added value of more sophisticated analysis methods. For example, the role of partial volume correction should be further investigated, as some studies have indicated GM/WM differences between autistic individuals and healthy controls [31,32].

In conclusion, our exploratory findings indicate that [18F]-FDOPA uptake in the brain does not significantly differ between autistic adults and controls. The striatal dopamine synthesis capacity can be negatively associated with scores on the AQ attention to detail subscale in autistic adults, but replication of this finding is necessary.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/2075-441 8/11/12/2404/s1. Table S1: Bayes factors (BF10) for the analyses examining striatal [18F]-FDOPA uptake (*k*icer min−1) in autistic adults and controls, and its association with self-reported autistic traits.

**Author Contributions:** Conceptualization, R.S., L.-F.d.G.-O., J.-P.S., T.v.A., J.B. and F.H.P.v.V.; Methodology, R.S., L.-F.d.G.-O., J.-P.S., M.Y., A.S., T.v.A., J.B. and F.H.P.v.V.; Software, R.S., F.H.P.v.V., A.S. and M.Y.; Validation, R.S., F.H.P.v.V., A.S. and M.Y.; Formal Analysis, R.S. and F.H.P.v.V.; Investigation, R.S., L.-F.d.G.-O. and F.H.P.v.V.; Resources, L.-F.d.G.-O., J.-P.S., M.Y., A.S., T.v.A., J.B. and F.H.P.v.V.; Data Curation, R.S., L.-F.d.G.-O., J.-P.S. and F.H.P.v.V.; Writing—Original Draft Preparation, R.S.; Writing—Review and Editing, L.-F.d.G.-O., J.-P.S., M.Y., A.S., T.v.A., J.B. and F.H.P.v.V.; Visualization, R.S., F.H.P.v.V., A.S. and M.Y.; Supervision, L.-F.d.G.-O., J.-P.S., J.B. and F.H.P.v.V.; Project Administration, R.S., L.-F.d.G.-O., J.-P.S. and F.H.P.v.V.; Funding Acquisition, L.-F.d.G.-O., J.-P.S. and F.H.P.v.V. All authors have read and agreed to the published version of the manuscript.

**Funding:** The study was funded in part by Stichting J.M.C. Kapteinfonds.

**Institutional Review Board Statement:** The study was conducted according to the guidelines of the Declaration of Helsinki, and approved by the Medical Ethical Committee of Leiden University Medical Center (reference NL54244.058.15; date of approval: 5 July 2016).

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.

**Data Availability Statement:** The datasets generated during and/or analyzed during the current study are available from the corresponding author upon reasonable request.

**Acknowledgments:** We would like to thank Jacqueline Aanholt-Bijlemeer, Ina Boot, Neanke Bouwman, Robert Bovenkerk, Johan van Brecht, Michael Bruijns, Paul de Bruin, Mark van Buchem, Petra Dibbets-Schneider, Demi Jansen, Jordi Vonk-van Oosten, and Patrick van der Zwet for facilitating the MRI and PET/CT scans. Furthermore, we would like to express our gratitude to Daniëlle Bos, Carlijn Clemens, Truda Driesen, Debora Op 't Eijnde, Erik Giltay, Jori Henke, and Jessie Kosterman for their assistance with conducting this study. We would also like to thank Ronald Boellaard, Patricia Cambraia Lopes, Elsmarieke van de Giessen, Sandeep Golla, Claus Svarer, and Charlotte van der Vos for their support with PET/CT data processing, and Fabian Termorshuizen for his support with the statistical analyses. Finally, we would like to thank JADOS (in particular Elles van Woerkum and Paul Stoffer), Anna Souverijn, Marcel Melchers, Els van der Ven, Villa Abel, PAS Nederland, Leviaan, aspergersyndroom.nl, autsider.net, RIAN Autismenetwerk, and the Dutch Association for Autism (Nederlandse Vereniging voor Autisme) for their help with recruiting study participants.

**Conflicts of Interest:** The authors declare no conflict of interest.
