Decreased thalamic monoamine availability in drug-induced parkinsonism

Drug-induced parkinsonism (DIP) is caused by a dopamine receptor blockade and is a major cause of misleading diagnosis of Parkinson’s disease (PD). Striatal dopamine activity has been investigated widely in DIP; however, most studies with dopamine transporter imaging have focused on the clinical characteristics and prognosis. This study investigated differences in striatal subregional monoamine availability among patients with DIP, normal controls, and patients with early PD. Thirty-five DIP patients, the same number of age-matched PD patients, and 46 healthy controls were selected for this study. Parkinsonian motor status was examined. Brain magnetic resonance imaging and positron emission tomography with 18F-N-(3-fluoropropyl)-2beta-carbon ethoxy-3beta-(4-iodophenyl) nortropane were performed, and the regional standardized uptake values were analyzed with a volume-of-interest template and compared among the groups. The groups were evenly matched for age, but there were numerically more females in the DIP group. Parkinsonian motor symptoms were similar in the DIP and PD groups. Monoamine availability in the thalamus of the DIP group was lower than that of the normal controls and similar to that of the PD group. In other subregions (putamen, globus pallidus, and ventral striatum), monoamine availability in the DIP group and normal controls did not differ and was higher than that in the PD group. This difference compared to healthy subject suggests that low monoamine availability in the thalamus could be an imaging biomarker of DIP.

and smoking did not differ among the three groups. The mean UPDRS total and part III scores in the DIP group were 20.0 ± 14.4, and 13.1 ± 8.1, respectively, which did not differ from the mean UPDRS total (23.6 ± 13.4, p = 0.283) and part III scores (14.0 ± 8.2, p = 0.642) in the PD group. The subscores of UPDRS part III including rest tremor (p = 0.120), rigidity (p = 0.584), and bradykinesia (p = 0.615) did not differ in DIP and PD group. The mean modified Hoehn and Yahr stage scores in the DIP group (2.0 ± 0.7) and PD group (1.8 ± 0.6) did not differ (p = 0.219) ( Table 1).
The SUVRs of the 18 F-FP-CIT PET images were analyzed. The SUVRs of all subregions in the PD group were lower than those in the normal control group. The DIP group had SUVRs similar to those of the normal controls and higher than those of the PD group in the putamen, globus pallidus, and ventral striatum. The SUVR of the thalamus in the DIP group (1.43 ± 0.19) was significantly lower than that in the normal control group after adjusting age and sex (1.62 ± 0.13, p < 0.001) ( Table 3, Fig. 1). These differences continued in the subgroup analysis when patients were further classified by sex (Supplementary Tables 1 and 2). UPDRS part III score and UPDRS total score were correlated with the SUVR values of putamen and globus pallidus in PD, whereas there were no correlations between UPDRS scores and striatal SUVR values in DIP (Supplementary Tables 3 and 4).

Discussion
This study demonstrated that patients with DIP had a decreased monoamine uptake pattern in the thalamus compared with normal controls. Striatal monoamine availability in the other subregions of DIP patients was similar to that in normal controls. Patients with PD had decreased striatal monoamine availability compared with normal controls.  indicates that a large differences exists between two groups. The colored region showed a significant area of p-value set as 0.01. The SUVR values in the thalamus of patients with DIP were lower than those of healthy controls. www.nature.com/scientificreports/ In this study, striatal monoamine availability was normal in DIP patients. Because parkinsonian symptoms in DIP patients are caused by blockade of post-synaptic dopamine receptors, dopamine transporter uptake should be normal in the striatum 5 . Although some DIP patients have reduced dopamine uptake, their subclinical parkinsonism was unmasked by anti-dopaminergic drugs and progressed after the offending drugs were stopped or their primary parkinsonism was worsened by the offending drug 4,5 . Our DIP patients had "pure" DIP, which means that their parkinsonian symptoms resolved after withdrawal of the offending drugs. Therefore, our study confirms that striatal monoamine availability in DIP is normal. However, our result differs from that of a recent study of dopamine transporters in symptomatic controls and healthy subjects that found symptomatic controls with essential tremor, vascular parkinsonism, or DIP to show higher putaminal dopamine transporter binding than healthy subjects 7 . Heterogeneity of symptomatic controls and selection bias in enrolling healthy subjects might explain that conflicting result.
In our study, only the SUVR of the thalamus differed significantly between the DIP group and normal controls. 18 F-FP-CIT is a radioligand that binds not only to the striatum, but also to extra-striatal subregions such as the thalamus, hypothalamus, and brainstem 8,9 . The binding properties of the 18 F-FP-CIT radioligand represent the availability of monoamine transporters such as dopamine, serotonin, and norepinephrine, and a decrease in its uptake implies impaired neural circuitry 8,9 . Serotonin is the main monoamine in the thalamus, and 18 F-FP-CIT uptake in the thalamus also signifies serotonergic transmission from brainstem 8 . Our study supports a previous study showing that PD patients had reduced thalamic serotonin transporter binding compared with normal controls 9 and reveals that DIP patients also have lower serotonin transporter binding in the thalamus than do normal controls.
The findings of this study are difficult to interpret. One possibility is that low thalamic serotoninergic availability is associated with increased risk of DIP. In PD, disruption of organized serotonergic control of the mesencephalic dopaminergic connections between the basal ganglia nuclei and across the basal ganglionic-corticothalamic circuits is involved in the onset of parkinsonian symptoms 10 . Therefore, the decreased monoamine uptake in the thalamus shown in this study could be associated with increased susceptibility to DIP upon exposure to an offending drug. Previous studies of the genetic polymorphism of serotonin receptors and transporters provide evidence of the development of DIP and extrapyramidal symptoms in a DIP patient and schizophrenic patients 11,12 . Additional explanation is that offending drugs such as selective serotonin reuptake inhibitors and many antipsychotics potentially block serotonin transporters in the thalamus 8 . Prokinetics act through the release of serotonin 5-hydroxytryptamine (5-HT), so the 5-HT 3 antagonism and 5-HT 4 agonism are the main mechanisms of prokinetics 13,14 . Mosapride 13 and levosulpiride 14 also act as 5-HT 3 antagonists. As transmission to the central nervous system is mediated predominantly by a 5-HT 3 antagonist 15 , serotonin uptake in the thalamus can be decreased by prokinetics. However, that hypothesis does not explain why these offending drugs do not affect presynaptic dopamine uptake in the striatum and affect the serotonin receptor only in the thalamus.
This study has several strengths and limitations. The major strength of our study is that we enrolled only patients with "pure" DIP whose symptoms resolved after withdrawal of the offending drug. DIP is a heterogeneous clinical syndrome; many patients have a full and long-lasting recovery with no subsequent PD, while other have persistent and worsening parkinsonian symptoms after discontinuation of the offending drug (subclinical parkinsonism or DIP unmasks PD) or recurrence of PD after complete remission from DIP (DIP antedates PD) 16 . The decreased striatal monoamine availability in DIP patients could be combined with subclinical PD and other atypical parkinsonism 4,5 . In addition, unlike most previous studies, we enrolled age-matched healthy subjects in our study. This study was conducted using a cross-sectional design and data from a PD registry, and we tried to match subject age using the similar patient matching method 17 . However, this study also has several limitations. First, we did not assess serotonergic binding uptake directly. Even though the 18 F-FP-CIT radioligand has high serotonin affinity in the extra-striatal thalamus, serotonin transporter imaging might have shown better results. In addition, as we did not perform a follow-up PET scan with DIP patients, alteration of thalamic monoamine availability could not be confirmed. Second, we could not match the sex ratio among groups. A recent study conducted age and sex correlation of dopamine transporter imaging, which showed putaminal dopamine availability was about 10% higher in female than male 18 . However, sex effect was weaker than age effect, and sex effect was most prominent at young age 18 . In this study of subjects, most subjects were aged over 60 years old, and therefore we can suppose sex effect of striatal monoamine availability might be minimized. Moreover, we also reconfirmed our finding in the subgroup analyses classified by sex.
To our knowledge, few studies have investigated quantitative differences in striatal monoamine availability among DIP patients, normal controls, and PD patients. The DIP patients had striatal monoamine availability similar to that of the normal controls and essentially normal striatal monoamine availability compared with PD patients. On the other hand, monoamine availability in the thalamus of DIP patients was lower than that in the normal controls and similar to that in PD patients. Low thalamic monoamine uptake is characteristic of DIP patients compared to normal controls, although its clinical significance needs to be further investigated. www.nature.com/scientificreports/ Subjects. Patients with diagnosed DIP who visited the movement disorder clinic in a university-affiliated hospital between January 2018 and December 2020 were enrolled. Age-matched PD patients newly diagnosed during the same period also were enrolled. DIP was defined using the following criteria: (1) presence of at least two of the four cardinal parkinsonian signs (tremor, rigidity, bradykinesia, and impaired postural reflexes); (2) absence of a personal history of extrapyramidal disorders before treatment with an offending drug; (3) onset of symptoms during the course of treatment with an offending drug; and (4) reversal of parkinsonian symptoms, although not necessarily completely, after discontinuation of the offending drug during follow-up of more than 6 months 19,20 . All DIP patients showed normal dopamine transporter uptake on a visual analysis of positron emission tomography (PET) with 18 F-N-(3-fluoropropyl)-2beta-carbon ethoxy-3beta-(4-iodophenyl) nortropane ( 18 F-FP-CIT). PD was diagnosed based on the UK Parkinson's Disease Society Brain Bank clinical diagnostic criteria 21 and the Movement Disorder Society clinical diagnostic criteria for PD 22 . All PD patients had decreased dopamine transporter uptake in the striatum, mainly in the posterior putamen on a visual analysis. PD patients were enrolled from our research registry using the similar patient matching method 17 . Forty-six healthy subjects without any notable neurological or psychiatric diseases were recruited and included as controls. All subjects underwent brain magnetic resonance imaging (MRI) and did not demonstrate any abnormalities beyond mild white matter changes. Demographics of age, sex, disease duration, medical history of hypertension and diabetes mellitus, and smoking status were collected. Parkinsonian motor symptoms were evaluated using the Unified Parkinson's Disease Rating Scale (UPDRS) 23 and modified Hoehn and Yahr stage score 24 . All patients underwent brain MRI and 18 F-FP-CIT PET at diagnosis. Normal subjects underwent brain MRI and 18 F-FP-CIT PET with the same protocol as used for patients.

Methods
Patients were excluded using the following criteria: (1) atypical or other secondary parkinsonism, (2) previous stroke or structural lesions on the basal ganglia, or (3) use of anti-parkinsonian medications or medications that influence striatal monoamine uptake.
Statistical Parametric Mapping 8 software (Wellcome Trust Centre for Neuroimaging, London, UK) implemented in MATLAB 2015a (MathWorks, Natick, MA, USA) was used for co-registration and spatial normalization of images and voxel-based comparisons. To spatially normalize the 18 F-FP-CIT PET images, an MR-guided conventional spatial normalization method was used 25 . Then, PET images were co-registered to individual MR images and spatially normalized to the Montreal Neurological Institute space using parameter normalizing, skull-stripped MR 25 . Volume of interest (VOI) templates of striatal subregions were obtained after subcortical parcellation and partial volume correction using FreeSurfer 5.1 (Massachusetts General Hospital, Harvard Medical School; http:// surfer. nmr. mgh. harva rd. edu). VOI templates of four striatal subregions, the thalamus, and the cerebellum were normalized spatially to the MR template. Then subregional uptake values for each side of the caudate, putamen, globus pallidus, ventral striatum, thalamus, and cerebellum were calculated using the in-house MATLAB 2015a program (MathWorks, Natick, MA, USA) from our previous studies [26][27][28] . The mean standardized uptake value ratio (SUVR) was calculated as target SUV divided by cerebellar SUV. www.nature.com/scientificreports/