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Reduced plasma serotonin and 5-hydroxyindoleacetic acid levels in Parkinson's disease are associated with nonmotor symptoms
Parkinsonism and Related Disorders xxx (2015) 1e6
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Parkinsonism and Related Disorders journal homepage: www.elsevier.com/locate/parkreldis
Reduced plasma serotonin and 5-hydroxyindoleacetic acid levels in Parkinson's disease are associated with nonmotor symptoms Qing Tong a, 1, Li Zhang a, 1, Yongsheng Yuan a, 1, Siming Jiang a, Rui Zhang b, Qinrong Xu a, Jian Ding a, Daqian Li c, Xiaobin Zhou d, Kezhong Zhang a, * a
Department of Neurology, The First Affiliated Hospital of Nanjing Medical University, Nanjing 210029, China Department of Neurosurgery, Nanjing Children's Hospital of Nanjing Medical University, Nanjing 210029, China Department of Laboratory Medicine, The First Affiliated Hospital of Nanjing Medical University, Nanjing 210029, China d Department of Microbiology and Immunology, Nanjing Medical University, Nanjing 210029, China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 January 2015 Received in revised form 3 May 2015 Accepted 25 May 2015
Background: Accumulating evidence suggests that serotonergic system may be implicated in the pathophysiology of Parkinson's disease (PD), and particularly in nonmotor symptoms such as depression, fatigue, sleep disorders, sensory and autonomic dysfunction. This study aimed to evaluate plasma levels of serotonin (5-HT) and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) in PD patients, and investigate their associations with nonmotor symptoms. Methods: Eighty-two PD patients and sixty-four controls underwent a series of clinical assessments, including Hamilton Depression Scale, Fatigue Severity Scale, Pittsburgh Sleep Quality Index, Visual Analog Scale for Pain, and Scale for Outcomes in PD for Autonomic Symptoms. Plasma 5-HT and 5-HIAA levels were measured by HPLC-ECD. Results: PD patients exhibited worse performance on nonmotor symptom scales (all P-values <0.001) and presented lower plasma levels of 5-HT (P < 0.001) and 5-HIAA (P < 0.001) than control individuals. Within the PD group, decreased concentrations of plasma 5-HT and 5-HIAA were correlated with more severe depression (r ¼ 0.447, P < 0.001; r ¼ 0.407, P < 0.001, respectively) and pain (r ¼ 0.485, P < 0.001; r ¼ 0.416, P < 0.001, respectively). After performing multiple linear regression, plasma 5-HT (P ¼ 0.01) and 5-HIAA (P ¼ 0.006) remained significantly associated with depression. Conclusions: Our results suggest that serotonergic dysfunction might exist in PD, and specifically correlated with depression and pain in PD. Plasma levels of 5-HT and 5-HIAA may be considered as peripheral markers for depression in PD. © 2015 Published by Elsevier Ltd.
Keywords: Serotonin 5-Hydroxyindoleacetic acid Depression Pain Parkinson's disease
1. Introduction Parkinson's disease (PD) is the second most common neurodegenerative disease, of which the main pathological hallmark is the loss of dopamine neurons in the substantia nigra. Clinically, PD is characterized by motor features including bradykinesia, rigidity, tremor, and postural instability. However, the classic view of PD as a pure dopaminergic and movement disorder is changing. Experimental and clinical findings have shown that serotonergic system might be involved in the pathophysiology of PD [1,2]. Additionally, a
* Corresponding author. Department of Neurology, The First Affiliated Hospital of Nanjing Medical University, No. 300 Guangzhou Road, Nanjing 210029, China. E-mail address: [email protected] (K. Zhang). 1 These authors contributed equally to this work.
wide spectrum of nonmotor symptoms (NMS), such as mood disturbances, fatigue, sleep disorders, sensory and autonomic dysfunction, also frequently occur in PD patients, which are considered to be disabling and contribute to poor quality of life [3,4]. The postmortem study has shown that the neuropathological process of PD extends beyond the striatum and dopaminergic system [5]. According to Braak's hypothesis, during the early stages the Lewy bodies occur in the raphe nuclei, which is responsible for the production of serotonin (5-HT), indicating a potential role of serotonergic system in the pathophysiology of PD [5]. Additional support for this concept comes from another postmortem study that observed the depletion of 5-HT and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) in PD patients [6]. Moreover, previous in vivo neuroimaging studies have
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Please cite this article in press as: Q. Tong, et al., Reduced plasma serotonin and 5-hydroxyindoleacetic acid levels in Parkinson's disease are associated with nonmotor symptoms, Parkinsonism and Related Disorders (2015), http://dx.doi.org/10.1016/j.parkreldis.2015.05.016
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Q. Tong et al. / Parkinsonism and Related Disorders xxx (2015) 1e6
reported significant reductions of serotonin transporter binding in the brainstem of PD patients [7,8]. Recently, Olivola et al. found the cerebrospinal fluid (CSF) levels of 5-HT and 5-HIAA in PD patients were markedly lower than those in controls [9]. Interestingly, there are data to suggest that serotonergic dysfunction may be implicated not only in PD, but also more specifically in the occurrence of NMS [10]. Emerging evidence from biochemical, animal models and human studies has demonstrated the involvement of serotonergic system in the pathophysiology of NMS in PD, including depression, fatigue, sleep disorders, pain, as well as autonomic dysfunction [11,12]. Furthermore, several open label studies have shown a beneficial effect of serotonergic agents on the treatment of motor and nonmotor symptoms in PD patients [2,13]. However, none of these studies attempted to investigate whether serotonin markers in plasma are altered in patients with PD and associated with such symptoms. Therefore, in this study, we sought to assess levels of 5-HT and 5-HIAA in plasma of PD patients, and evaluate their associations with NMS, which in turn may provide evidence for the use of agents acting on serotonergic neurotransmission. 2. Methods 2.1. Patients Eighty-two patients with idiopathic PD, including newly diagnosed and advanced patients, were recruited consecutively from the Department of Neurology of the First Affiliated Hospital of Nanjing Medical University, China, between August 2012 and June 2013. All patients were diagnosed by two experienced neurologists according to the UK Parkinson's Disease Society Brain Bank criteria, with the disease duration ranged from 0.5 to 18 years. Exclusion criteria were: neurological diseases other than PD, severe hearing or visual loss, or other conditions that might interfere with the reliable completion of clinical assessments. As control group, sixty-four age- and gender-matched individuals without a family history of PD and neurological diseases were also recruited from our institution. Both patients and controls were included only if they were volunteered to participate in scientific studies, and if no signs of cognitive impairments were detectable by the Mini-Mental State Examination (MMSE score <24). PD patients included in our study had a stable response to their antiparkinsonian medications. Moreover, none of the participants was taking medications acting on serotonergic system. This study was approved by the ethics committee of the First Affiliated Hospital of Nanjing Medical University and all subjects provided written informed consent. 2.2. Clinical assessment All patients were evaluated using the Unified Parkinson's Disease Rating Scale (UPDRS) and the Hoehn and Yahr staging scale (H&Y) during their off state. In addition, Levodopa-equivalent daily dose (LEDD), which served to establish the anti-parkinsonian medications, was calculated according to conversion factors as described previously [14]. All study participants were subjected to a thorough clinical assessment, which included validated measures for the evaluation of NMS in PD patients. The severity of depressive symptoms was assessed with the Hamilton Depression Scale (HAMD), in which the cut-off value for screening purposes is 9/10 [15]. Fatigue was evaluated by using the Fatigue Severity Scale (FSS) and patients with a score 4 were defined as suffering from significant fatigue. Sleep disorder was assessed using the Pittsburgh Sleep Quality Index (PSQI), with a score > 5 indicating the presence of sleep disturbance. The pain severity was recorded with the
Visual Analog Scale (VAS) for Pain. Finally, the autonomic dysfunction was evaluated using the Scale for Outcomes in PD for Autonomic Symptoms (SCOPA-AUT). 2.3. Plasma sampling and biological assays Venous blood samples were drawn in vacuum tubes containing EDTA in the morning on the same day of clinical assessment, after an overnight fast. Considering the full vanishing of the effect of prolonged released dopamine agonists, venipuncture was conducted in PD patients after three-day withdrawal of anti-parkinsonian medications. From another aspect, five days or more was considered to be unethical by the ethics committee. Blood samples were immediately centrifuged at 3,000 g for 15 min at 4 C, and plasma was aliquoted and stored at 80 C until analysis. At the time of analysis, all plasma samples were deproteinized with trichloroacetic acid (1: 0.24 V/ V) after thawing, and immediately centrifuged at 20,000 g for 30 min at 4 C. The protein-free supernatants were then measured by HPLC-ECD. Briefly, the HPLC-ECD system consisted of a Thermo UniMate 3000 pump, an UniMate 3000 autosampler, and an ESA Coulochem III electrochemical detector (working electrode þ350mv). Thermo BDS HYPERSIL column (4.6 250 mm, 5 mm) was used at 38 C. The mobile phase (90 mmol/L NaH2PO4, 50 mmol/L citric acid, 1.7 mmol/L OSA, 50umol/L EDTA, 10% acetonitrile) was delivered to the analytical column at a rate of 1.0 ml/min. Chromeleon 6.8 software was used for data collection and processing. 2.4. Statistical analysis Statistical analyses were performed with SPSS software (version 19.0, SPSS Inc., Chicago, IL, USA). All data were tested for normality of distribution by using ShapiroeWilk test (P > 0.05). Comparisons between groups were made using Chi-squared test for categorical
Table 1 Demographic and clinical features of PD patients and control subjects.
Age (years), mean ± SD Male: females Disease duration (years), mean ± SD UPDRS score, mean ± SD UPDRS I score, mean ± SD UPDRS II score, mean ± SD UPDRS III score, mean ± SD Hoehn and Yahr stage, mean ± SD LEDD (mg/day), mean ± SD MMSE score, mean ± SD HAMD score, mean ± SD FSS score, mean ± SD PSQI score, mean ± SD VAS score, mean ± SD SCOPA-AUT score, mean ± SD 5-HT (mg/L), mean ± SD 5-HIAA (mg/L), mean ± SD
PD (n ¼ 82)
Control (n ¼ 64)
P
60.88 ± 10.77 29:53 3.35 ± 3.62
63.92 ± 10.60 28:36 e
0.09a 0.303b e
27.67 ± 13.60 2.66 ± 1.83 11.28 ± 6.30 13.85 ± 7.75 1.82 ± 0.76
e e e e e
e e e e e
201.14 ± 331.72 27.35 ± 3.14 11.13 ± 7.59 4.19 ± 1.60 5.78 ± 3.74 3.87 ± 2.25 11.68 ± 9.23 26.53 ± 15.07 96.48 ± 51.70
e 28.11 ± 2.85 0.86 ± 0.94 1.93 ± 0.68 0.31 ± 0.59 0.39 ± 0.88 0.64 ± 0.97 42.58 ± 16.99 186.75 ± 99.12
e 0.357c <0.001c <0.001c <0.001c <0.001c <0.001c <0.001c <0.001c
PD, Parkinson's disease; SD, Standard deviation; UPDRS, Unified Parkinson's disease rating scale; LEDD, levodopa-equivalent daily dose; MMSE, Mini-Mental State Examination; HAMD, Hamilton Depression Scale; FSS, Fatigue Severity Scale; PSQI, Pittsburgh Sleep Quality Index; VAS, Visual Analog Scale for Pain; SCOPA-AUT, Scale for Outcomes in PD for Autonomic Symptoms; 5-HT, serotonin; 5-HIAA, 5hydroxyindolacetic acid. a Student's t-test. b Chi-square test. c ManneWhitney test.
Please cite this article in press as: Q. Tong, et al., Reduced plasma serotonin and 5-hydroxyindoleacetic acid levels in Parkinson's disease are associated with nonmotor symptoms, Parkinsonism and Related Disorders (2015), http://dx.doi.org/10.1016/j.parkreldis.2015.05.016
Q. Tong et al. / Parkinsonism and Related Disorders xxx (2015) 1e6
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3. Results 3.1. Demographic and clinical characteristics of PD patients and controls
Fig. 1. Plasma levels of 5-HT and 5-HIAA in PD patients and controls. PD patients presented lower plasma levels of 5-HT and 5-HIAA than control individuals. ***P < 0.001; PD, Parkinson's disease; 5-HT, serotonin; 5-HIAA, 5-hydroxyindoleacetic acid.
data, and independent samples t-test or ManneWhitney U test for continuous variables as appropriate. Spearman's rank correlation analyses were conducted to evaluate the correlation between clinical variables and plasma levels of 5-HT and 5-HIAA. In order to further examine the impact of serotonin markers on NMS in PD, the multiple linear regression analysis was subsequently performed. Clinical and demographic features likely to affect NMS (gender, age, disease duration, UPDRS III score, H&Y stage and LEDD), as well as plasma levels of 5-HT and 5-HIAA were entered as independent variables in each regression model using respective NMS score as the dependent variable. P values less than 0.05 were considered statistically significant.
Demographic and clinical characteristics of all subjects are shown in Table 1. Eighty-two PD patients and sixty-four age- and gender-matched controls were recruited in this study. We found that NMS occurred frequently in Chinese PD patients: depression (48 patients), fatigue (43 patients), sleep disturbance (40 patients), pain (56 patients), autonomic dysfunction (62 patients). In the PD group, disease duration was significantly associated with HAMD score (r ¼ 0.272, P ¼ 0.013), FSS score (r ¼ 0.257, P ¼ 0.020) and VAS score (r ¼ 0.357, P ¼ 0.001). The severity and stage of PD, as assessed by UPDRS III score and H&Y stage were positively correlated with HAMD score (r ¼ 0.302, P ¼ 0.006; r ¼ 0.298, P ¼ 0.007, respectively). Moreover, LEDD was markedly associated with VAS score (r ¼ 0.391, P < 0.001) and SCOPA-AUT score (r ¼ 0.40, P < 0.001), while weakly correlated with PSQI score (r ¼ 0.272, P ¼ 0.014). 3.2. Plasma levels of 5-HT and 5-HIAA PD patients presented significantly lower plasma levels of 5-HT and 5-HIAA compared to control individuals (Table 1, Fig. 1). Within the PD group, as illustrated in Fig. 2, the decreased levels of plasma 5-HT and 5-HIAA were correlated with the severity of depression as evaluated by the HAMD score (r ¼ 0.447, P < 0.001; r ¼ 0.407, P < 0.001, respectively). Similarly, plasma levels of 5-HT and 5-HIAA were also negatively associated with VAS score (r ¼ 0.485, P < 0.001; r ¼ 0.416, P < 0.001, respectively). Nevertheless, we failed to observe any significant correlations between reduced levels of plasma 5-HT and 5-HIAA with the disease duration and
Fig. 2. Correlations between plasma levels of 5-HT and 5-HIAA and NMS in PD patients. (a): Plasma 5-HT level significantly correlated with HAMD score (r ¼ 0.447, r2 ¼ 0.207, P < 0.001); (b): Plasma 5-HIAA level significantly correlated with HAMD score (r ¼ 0.407, r2 ¼ 0.214, P < 0.001); (c): Plasma 5-HT level significantly correlated with VAS score (r ¼ 0.485, r2 ¼ 0.260, P < 0.001); (d): Plasma 5-HIAA level significantly correlated with VAS score (r ¼ 0.416, r2 ¼ 0.206, P < 0.001).
Please cite this article in press as: Q. Tong, et al., Reduced plasma serotonin and 5-hydroxyindoleacetic acid levels in Parkinson's disease are associated with nonmotor symptoms, Parkinsonism and Related Disorders (2015), http://dx.doi.org/10.1016/j.parkreldis.2015.05.016
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Table 2 Multiple linear regression analysis with HAMD score as the dependent variable. Variables
Unstandardized coefficients (B)
SE
Standardized coefficients(b)
P
Gender Age Disease duration UPDRS III Hoehn and Yahr stage LEDD 5-HT 5-HIAA
3.844 0.028 0.352 0.236 0.058 0.003 0.143 0.044
1.454 0.067 0.244 0.123 1.333 0.003 0.054 0.015
0.244 0.040 0.168 0.244 0.006 0.148 0.284 0.300
0.010* 0.671 0.153 0.059 0.965 0.201 0.010* 0.006*
Adjusted R2
0.341(P < 0.001)* HAMD, Hamilton Depression Scale; SE, standard error of B; UPDRS, Unified Parkinson's disease rating scale; LEDD, levodopa-equivalent daily dose; 5-HT, serotonin; 5-HIAA, 5hydroxyindoleacetic acid; *P < 0.05.
staging, the UPDRS score and its subtests, and the antiparkinsonian medication intake, as well as other NMS assessed in this study, including fatigue, sleep disorders and autonomic dysfunction. In the control group, no significant correlations were found between plasma 5-HT and 5-HIAA concentrations and NMS. 3.3. Multiple linear regression analysis A multiple linear regression analysis with HAMD score as the dependent variable was carried out to evaluate the contribution of the factors including gender, age, disease duration, UPDRS III score, H&Y stage and LEDD, as well as plasma levels of 5-HT and 5-HIAA in HAMD (Table 2). The regression model was significant (P < 0.001) and accounted for 34.1% of variance in HAMD score. Moreover, plasma levels of 5-HT (b ¼ 0.284, P ¼ 0.010) and 5-HIAA (b ¼ 0.300, P ¼ 0.006), and gender (b ¼ 0.244, P ¼ 0.010) were found to be significantly correlated with HAMD score. Although the levels of plasma 5-HT and 5-HIAA negatively correlated with VAS score, as previously noted, the multiple regression analysis with VAS score as the dependent variable was not performed due to its non-normality even after several attempts at data transformation. 4. Discussion Over the last decade, accumulating evidence from postmortem, biochemical and neuroimaging studies suggests that serotonergic dysfunction, as measured in brain tissue and CSF, is involved in motor and nonmotor symptoms of PD, including depression, fatigue, pain, sleep and autonomic impairment [2,10,11,16]. However, less systematic work has been conducted to address the potential roles of 5-HT and 5-HIAA in PD by measuring their plasma levels. Emerging data from animal models and human studies have shown that concentrations of both 5-HT and 5-HIAA in plasma were strongly correlated with those in the brain tissue and CSF samples [17e19], suggesting that the plasma serotonin markers might partially reflect the serotonergic activity in the brain. Moreover, the procedure for collecting plasma samples is less invasive and much safer for patients compared to the CSF samples. Based on these data, we sought to determine levels of plasma 5-HT and 5-HIAA in PD patients, and investigate their associations with NMS. In this study, we observed that both plasma 5-HT and 5-HIAA levels were significantly decreased in PD patients, suggesting the presence of serotonergic dysfunction in PD. In line with our findings, previous studies have also shown decreased concentrations of 5-HT and 5-HIAA in brain and CSF of PD patients [6,9]. Moreover, striatal levels of the crucial serotonin markers, including 5-HT and 5-HIAA, as well as their transporter protein and synthesizing enzyme protein, were reduced in postmortem studies, providing compelling evidence for the involvement of serotonergic dysfunction in PD [1,20]. Consistently, several in vivo imaging studies have
confirmed some of these observations by showing a marked decrease of 5-HT transporter binding in the striatum of PD patients [7,8]. Taken together, these data support a crucial role of serotonergic dysfunction in the pathophysiology of PD. However, Engelborghs et al. found the CSF level of 5-HT was significantly higher in PD patients [21]. Additionally, a previous positron emission tomography (PET) study performed in nine patients with early PD has indicated that 5-HT transporter binding was not reduced in patients compared to healthy controls [22]. These variable findings might be attributable to differences in sample characteristics, different neurochemical analysis techniques, and differences in antiparkinsonian medications. Although the link between dopaminergic treatment and serotonergic dysfunction has yet to be discerned, several studies have suggested that chronic exposure to dopaminergic therapy might affect the serotonergic system in PD. For example, Chia et al. found a significant reduction of plasma 5-HT in PD patients receiving levodopa (L-DOPA) treatment [23]. Also, data from parkinsonian rats have suggested that chronic L-DOPA therapy could dramatically reduced striatal levels of 5-HT and 5HIAA [24]. These results were supported by an in vivo neurochemical study showing markedly reduced basal 5-HT release and metabolism in rat models of PD treated with a therapeutic dose of LDOPA [25]. However, in this study, we did not find any correlations between LEDD and plasma levels of 5-HT and 5-HIAA. A possible explanation for this discrepancy might be that all patients in the present study, in line with previous studies concerning serotonergic neurotransmission in PD [9,11,26], had their anti-parkinsonian medications stopped for 3 days before plasma sampling. To our knowledge, this is the first study to investigate the relationship between plasma levels of serotonin markers and NMS in PD patients. Although the pathophysiological mechanisms underlying NMS are complex and remain to be determined, converging evidence supports the concept that serotonergic dysfunction might be implicated in the occurrence of NMS. Based on Braak's hypothesis [5], serotonergic system that was thought to modulate several physiologic functions, such as emotion, sleep and wakefulness, and cognition, was affected in the neurodegenerative process of PD, suggesting a potential role of serotonergic pathway in NMS of PD. Mayeux et al. found the decreased CSF level of 5-HIAA was related to the presence of depression in PD patients [27]. Furthermore, a PET study has shown a significant reduction of 5-HT tracer uptake in the limbic system in depressed PD patients versus nondepressed patients [28]. In agreement with these observations, our current findings demonstrated negative associations between plasma levels of serotonin markers and the severity of depression in PD patients. However, previous studies have suggested that clinical characteristics such as gender, age, disease duration, and the disease severity and stage may be associated with depression in PD [29,30]. As such, in this study, we tried to keep all these confounding factors to a minimum by performing multiple linear regressions. After controlling for covariates, the decreased plasma
Please cite this article in press as: Q. Tong, et al., Reduced plasma serotonin and 5-hydroxyindoleacetic acid levels in Parkinson's disease are associated with nonmotor symptoms, Parkinsonism and Related Disorders (2015), http://dx.doi.org/10.1016/j.parkreldis.2015.05.016
Q. Tong et al. / Parkinsonism and Related Disorders xxx (2015) 1e6
levels of 5-HT and 5-HIAA remained significantly associated with the severity of depression, which suggests that the altered serotonergic function might contribute to the genesis of depression in PD, and plasma levels of 5-HT and 5-HIAA may be considered as peripheral markers for depression of PD. Additionally, the findings from this study may provide evidence for the use of selective serotonin reuptake inhibitors (SSRI) as antidepressants in PD. In support, a placebo-controlled study has demonstrated that paroxetine significantly improved depressive symptoms in PD patients, which showed marked reductions of HAMD scores with drugs [31]. Nevertheless, these results should be taken cautiously as significant treatment effects of paroxetine were not observed in another placebo-controlled clinical trial [32]. Thus, further research aimed at evaluating the efficacy of serotonergic agents in depression of PD, with large sample size and long-duration, is warranted. However, there are inconsistent findings for the involvement of serotonergic dysfunction in the pathophysiology of depression in PD patients. For example, Olivola et al. found the reduced level of CSF 5-HT in PD patients was not associated with the presence of depression [9]. Additionally, a PET imaging study observed increased 5-HT transporter binding in depressed PD patients compared to healthy controls [33]. These discrepancies between studies may be due to the differences in sample size, the different methods of biological analysis and inclusion criteria. Compared with prior work, we recruited a relatively larger sample size of consecutive PD patients, of all disease duration and stage, and assessed the concentrations of serotonin markers in plasma samples by using a more sensitive and stable measurement. It is noteworthy that our study also observed the link between plasma levels of serotonin markers and the severity of pain in PD patients. In line with our findings, data from an animal study have revealed decreased 5-HT content of dialysates in contralateral ventrobasal thalamus and raphe magnus nucleus in rat models of neuropathic pain [34]. Recently, a clinical and neuroimaging study found that apomorphine has no influence on pain thresholds or pain-induced cerebral activity in PD patients as compared with placebo, providing evidence for the involvement of other nondopaminergic systems such as norepinephrine and serotonin in pain in PD [35]. Actually, the role of serotonergic system in pain modulation in PD has been confirmed by a study showing duloxetine, the selective 5-HT reuptake inhibitor, seems to be beneficial for the improvement of pain in PD patients [36]. Despite the fact that VAS score in our study was non-normally distributed and limited the following statistical analysis, our findings indicate that serotonergic dysfunction might contribute to the pathogenesis of pain in PD. Of note, we are aware that there are some limitations in this study. First, we performed a peripheral assessment of serotonergic dysfunction by evaluating 5-HT and 5-HIAA in plasma samples; second, the sample size of our study was relative small for correlation analysis; third, our work was restricted to five types of NMS in PD patients, and consequently ignored other NMS such as cognitive impairments. Therefore, larger scale studies employing multiple approaches such as PET are still indispensable to elucidate the role of serotonergic dysfunction in PD, particularly with respect to NMS, as they may provide crucial evidence for the treatment. In conclusion, our study showed that PD patients exhibited lower plasma levels of 5-HT and 5-HIAA than control individuals, and the decreased plasma serotonin markers were correlated with the severity of depression and pain. Combined with the findings from previous work, our results suggest that serotonergic dysfunction may contribute to nonmotor symptoms in PD patients, and plasma levels of serotonin markers would be considered as peripheral markers for depression in PD.
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Conflicts of interest The authors declare no conflict of interest. Acknowledgments This work was supported by University Natural Science Research Project in Jiangsu Province (No.13KJB32009), Natural Science Foundation of Jiangsu Province (No. BK20141494), Opening Project of Jiangsu Key Laboratory of Neurodegeneration (No. SJ11KF01), Jiangsu Provincial Personnel Department “the Great of Six Talented Man Peak” (No.2014-WSN-013), and Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). References [1] P. Huot, S.H. Fox, J.M. Brotchie, The serotonergic system in Parkinson's disease, Prog. Neurobiol. 95 (2011) 163e212. [2] S.H. Fox, R. Chuang, J.M. Brotchie, Serotonin and Parkinson's disease: on movement, mood, and madness, Mov. Disord. 24 (2009) 1255e1266. [3] T.K. Khoo, A.J. Yarnall, G.W. Duncan, S. Coleman, J.T. O'Brien, D.J. Brooks, et al., The spectrum of nonmotor symptoms in early Parkinson disease, Neurology 80 (2013) 276e281. [4] G.W. Duncan, T.K. Khoo, A.J. Yarnall, J.T. O'Brien, S.Y. Coleman, D.J. Brooks, et al., Health-related quality of life in early Parkinson's disease: the impact of nonmotor symptoms, Mov. Disord. 29 (2014) 195e202. [5] H. Braak, K.D. Tredici, U. Rüb, R.A. de Vos, E.N. Jansen Steur, E. Braak, Staging of brain pathology related to sporadic Parkinson's disease, Neurobiol. Aging 24 (2003) 197e211. [6] K. Shannak, A. Rajput, B. Rozdilsky, S. Kish, J. Gilbert, O. Hornykiewicz, Noradrenaline, dopamine and serotonin levels and metabolism in the human hypothalamus: observations in Parkinson's disease and normal subjects, Brain Res. 639 (1994) 33e41. [7] M. Guttman, I. Boileau, J. Warsh, J.A. Saint-Cyr, N. Ginovart, T. McCluskey, et al., Brain serotonin transporter binding in non-depressed patients with Parkinson's disease, Eur. J. Neurol. 14 (2007) 523e528. [8] L. Kerenyi, G.A. Ricaurte, D.J. Schretlen, U. McCann, J. Varga, W.B. Mathews, et al., Positron emission tomography of striatal serotonin transporters in Parkinson disease, Arch. Neurol. 60 (2003) 1223e1229. [9] E. Olivola, M. Pierantozzi, P. Imbriani, C. Liguori, M.S. Bassi, M. Conti, et al., Serotonin Impairment in CSF of PD Patients, without an Apparent Clinical Counterpart, PLoS One 9 (2014). [10] T. Simuni, K. Sethi, Nonmotor manifestations of Parkinson's disease, Ann. Neurology 64 (Suppl 2) (2008) S65eS80. [11] M. Politis, K. Wu, C. Loane, N.P. Quinn, D.J. Brooks, W.H. Oertel, et al., Serotonin neuron loss and nonmotor symptoms continue in Parkinson's patients treated with dopamine grafts, Sci. Transl. Med. 4 (2012) 128e141. [12] M. Politis, C. Loane, Serotonergic dysfunction in Parkinson's disease and its relevance to disability, ScientificWorldJournal 11 (2011) 1726e1734. [13] K. Seppi, D. Weintraub, M. Coelho, S. Perez-Lloret, S.H. Fox, R. Katzenschlager, et al., The movement disorder society evidence-based medicine review update: treatments for the non-motor symptoms of Parkinson's disease, Mov. Disord. 26 (Suppl. 3) (2011) S42eS80. [14] C.L. Tomlinson, R. Stowe, S. Patel, C. Rick, R. Gray, C.E. Clarke, Systematic review of levodopa dose equivalency reporting in Parkinson's disease, Mov. Disord. 25 (2010) 2649e2653. [15] A. Schrag, P. Barone, R.G. Brown, A.F. Leentjens, W.M. McDonald, S. Starkstein, et al., Depression rating scales in Parkinson's disease: critique and recommendations, Mov. Disord. 22 (2007) 1077e1092. [16] P. Huot, S.H. Fox, The serotonergic system in motor and non-motor manifestations of Parkinson's disease, Exp. Brain Res. 230 (2013) 463e476. [17] M.J. Sarrias, P. Cabre, E. Martinez, F. Artigas, Relationship between serotoninergic measures in blood and cerebrospinal fluid simultaneously obtained in humans, J. Neurochem. 54 (1990) 783e786. [18] M. Liu, J. Xiang, J. Zhou, H. Ding, A disposable amperometric sensor for rapid detection of serotonin in the blood and brain of the depressed mice based on Nafion membrane-coated colloidal gold screen-printed electrode, J. Electroanal. Chem. 640 (2010) 1e7. [19] T. Audhya, J.B. Adams, L. Johansen, Correlation of Serotonin Levels in CSF, Platelets, Plasma, and Urine. Biochimica et Biophysica Acta (BBA)-General Subjects 2012, 1820, pp. 1496e1501. [20] S.J. Kish, J. Tong, O. Hornykiewicz, A. Rajput, L.J. Chang, M. Guttman, et al., Preferential loss of serotonin markers in caudate versus putamen in Parkinson's disease, Brain 131 (2008) 120e131. [21] S. Engelborghs, B. Marescau, P.P. De Deyn, Amino acids and biogenic amines in cerebrospinal fluid of patients with Parkinson's disease, Neurochem. Res. 28 (2003) 1145e1150. [22] K. Strecker, F. Wegner, S. Hesse, G.A. Becker, M. Patt, P.M. Meyer, et al., Preserved serotonin transporter binding in de novo Parkinson's disease: negative
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Contents lists available at ScienceDirect
Parkinsonism and Related Disorders journal homepage: www.elsevier.com/locate/parkreldis
Reduced plasma serotonin and 5-hydroxyindoleacetic acid levels in Parkinson's disease are associated with nonmotor symptoms Qing Tong a, 1, Li Zhang a, 1, Yongsheng Yuan a, 1, Siming Jiang a, Rui Zhang b, Qinrong Xu a, Jian Ding a, Daqian Li c, Xiaobin Zhou d, Kezhong Zhang a, * a
Department of Neurology, The First Affiliated Hospital of Nanjing Medical University, Nanjing 210029, China Department of Neurosurgery, Nanjing Children's Hospital of Nanjing Medical University, Nanjing 210029, China Department of Laboratory Medicine, The First Affiliated Hospital of Nanjing Medical University, Nanjing 210029, China d Department of Microbiology and Immunology, Nanjing Medical University, Nanjing 210029, China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 January 2015 Received in revised form 3 May 2015 Accepted 25 May 2015
Background: Accumulating evidence suggests that serotonergic system may be implicated in the pathophysiology of Parkinson's disease (PD), and particularly in nonmotor symptoms such as depression, fatigue, sleep disorders, sensory and autonomic dysfunction. This study aimed to evaluate plasma levels of serotonin (5-HT) and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) in PD patients, and investigate their associations with nonmotor symptoms. Methods: Eighty-two PD patients and sixty-four controls underwent a series of clinical assessments, including Hamilton Depression Scale, Fatigue Severity Scale, Pittsburgh Sleep Quality Index, Visual Analog Scale for Pain, and Scale for Outcomes in PD for Autonomic Symptoms. Plasma 5-HT and 5-HIAA levels were measured by HPLC-ECD. Results: PD patients exhibited worse performance on nonmotor symptom scales (all P-values <0.001) and presented lower plasma levels of 5-HT (P < 0.001) and 5-HIAA (P < 0.001) than control individuals. Within the PD group, decreased concentrations of plasma 5-HT and 5-HIAA were correlated with more severe depression (r ¼ 0.447, P < 0.001; r ¼ 0.407, P < 0.001, respectively) and pain (r ¼ 0.485, P < 0.001; r ¼ 0.416, P < 0.001, respectively). After performing multiple linear regression, plasma 5-HT (P ¼ 0.01) and 5-HIAA (P ¼ 0.006) remained significantly associated with depression. Conclusions: Our results suggest that serotonergic dysfunction might exist in PD, and specifically correlated with depression and pain in PD. Plasma levels of 5-HT and 5-HIAA may be considered as peripheral markers for depression in PD. © 2015 Published by Elsevier Ltd.
Keywords: Serotonin 5-Hydroxyindoleacetic acid Depression Pain Parkinson's disease
1. Introduction Parkinson's disease (PD) is the second most common neurodegenerative disease, of which the main pathological hallmark is the loss of dopamine neurons in the substantia nigra. Clinically, PD is characterized by motor features including bradykinesia, rigidity, tremor, and postural instability. However, the classic view of PD as a pure dopaminergic and movement disorder is changing. Experimental and clinical findings have shown that serotonergic system might be involved in the pathophysiology of PD [1,2]. Additionally, a
* Corresponding author. Department of Neurology, The First Affiliated Hospital of Nanjing Medical University, No. 300 Guangzhou Road, Nanjing 210029, China. E-mail address: [email protected] (K. Zhang). 1 These authors contributed equally to this work.
wide spectrum of nonmotor symptoms (NMS), such as mood disturbances, fatigue, sleep disorders, sensory and autonomic dysfunction, also frequently occur in PD patients, which are considered to be disabling and contribute to poor quality of life [3,4]. The postmortem study has shown that the neuropathological process of PD extends beyond the striatum and dopaminergic system [5]. According to Braak's hypothesis, during the early stages the Lewy bodies occur in the raphe nuclei, which is responsible for the production of serotonin (5-HT), indicating a potential role of serotonergic system in the pathophysiology of PD [5]. Additional support for this concept comes from another postmortem study that observed the depletion of 5-HT and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) in PD patients [6]. Moreover, previous in vivo neuroimaging studies have
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reported significant reductions of serotonin transporter binding in the brainstem of PD patients [7,8]. Recently, Olivola et al. found the cerebrospinal fluid (CSF) levels of 5-HT and 5-HIAA in PD patients were markedly lower than those in controls [9]. Interestingly, there are data to suggest that serotonergic dysfunction may be implicated not only in PD, but also more specifically in the occurrence of NMS [10]. Emerging evidence from biochemical, animal models and human studies has demonstrated the involvement of serotonergic system in the pathophysiology of NMS in PD, including depression, fatigue, sleep disorders, pain, as well as autonomic dysfunction [11,12]. Furthermore, several open label studies have shown a beneficial effect of serotonergic agents on the treatment of motor and nonmotor symptoms in PD patients [2,13]. However, none of these studies attempted to investigate whether serotonin markers in plasma are altered in patients with PD and associated with such symptoms. Therefore, in this study, we sought to assess levels of 5-HT and 5-HIAA in plasma of PD patients, and evaluate their associations with NMS, which in turn may provide evidence for the use of agents acting on serotonergic neurotransmission. 2. Methods 2.1. Patients Eighty-two patients with idiopathic PD, including newly diagnosed and advanced patients, were recruited consecutively from the Department of Neurology of the First Affiliated Hospital of Nanjing Medical University, China, between August 2012 and June 2013. All patients were diagnosed by two experienced neurologists according to the UK Parkinson's Disease Society Brain Bank criteria, with the disease duration ranged from 0.5 to 18 years. Exclusion criteria were: neurological diseases other than PD, severe hearing or visual loss, or other conditions that might interfere with the reliable completion of clinical assessments. As control group, sixty-four age- and gender-matched individuals without a family history of PD and neurological diseases were also recruited from our institution. Both patients and controls were included only if they were volunteered to participate in scientific studies, and if no signs of cognitive impairments were detectable by the Mini-Mental State Examination (MMSE score <24). PD patients included in our study had a stable response to their antiparkinsonian medications. Moreover, none of the participants was taking medications acting on serotonergic system. This study was approved by the ethics committee of the First Affiliated Hospital of Nanjing Medical University and all subjects provided written informed consent. 2.2. Clinical assessment All patients were evaluated using the Unified Parkinson's Disease Rating Scale (UPDRS) and the Hoehn and Yahr staging scale (H&Y) during their off state. In addition, Levodopa-equivalent daily dose (LEDD), which served to establish the anti-parkinsonian medications, was calculated according to conversion factors as described previously [14]. All study participants were subjected to a thorough clinical assessment, which included validated measures for the evaluation of NMS in PD patients. The severity of depressive symptoms was assessed with the Hamilton Depression Scale (HAMD), in which the cut-off value for screening purposes is 9/10 [15]. Fatigue was evaluated by using the Fatigue Severity Scale (FSS) and patients with a score 4 were defined as suffering from significant fatigue. Sleep disorder was assessed using the Pittsburgh Sleep Quality Index (PSQI), with a score > 5 indicating the presence of sleep disturbance. The pain severity was recorded with the
Visual Analog Scale (VAS) for Pain. Finally, the autonomic dysfunction was evaluated using the Scale for Outcomes in PD for Autonomic Symptoms (SCOPA-AUT). 2.3. Plasma sampling and biological assays Venous blood samples were drawn in vacuum tubes containing EDTA in the morning on the same day of clinical assessment, after an overnight fast. Considering the full vanishing of the effect of prolonged released dopamine agonists, venipuncture was conducted in PD patients after three-day withdrawal of anti-parkinsonian medications. From another aspect, five days or more was considered to be unethical by the ethics committee. Blood samples were immediately centrifuged at 3,000 g for 15 min at 4 C, and plasma was aliquoted and stored at 80 C until analysis. At the time of analysis, all plasma samples were deproteinized with trichloroacetic acid (1: 0.24 V/ V) after thawing, and immediately centrifuged at 20,000 g for 30 min at 4 C. The protein-free supernatants were then measured by HPLC-ECD. Briefly, the HPLC-ECD system consisted of a Thermo UniMate 3000 pump, an UniMate 3000 autosampler, and an ESA Coulochem III electrochemical detector (working electrode þ350mv). Thermo BDS HYPERSIL column (4.6 250 mm, 5 mm) was used at 38 C. The mobile phase (90 mmol/L NaH2PO4, 50 mmol/L citric acid, 1.7 mmol/L OSA, 50umol/L EDTA, 10% acetonitrile) was delivered to the analytical column at a rate of 1.0 ml/min. Chromeleon 6.8 software was used for data collection and processing. 2.4. Statistical analysis Statistical analyses were performed with SPSS software (version 19.0, SPSS Inc., Chicago, IL, USA). All data were tested for normality of distribution by using ShapiroeWilk test (P > 0.05). Comparisons between groups were made using Chi-squared test for categorical
Table 1 Demographic and clinical features of PD patients and control subjects.
Age (years), mean ± SD Male: females Disease duration (years), mean ± SD UPDRS score, mean ± SD UPDRS I score, mean ± SD UPDRS II score, mean ± SD UPDRS III score, mean ± SD Hoehn and Yahr stage, mean ± SD LEDD (mg/day), mean ± SD MMSE score, mean ± SD HAMD score, mean ± SD FSS score, mean ± SD PSQI score, mean ± SD VAS score, mean ± SD SCOPA-AUT score, mean ± SD 5-HT (mg/L), mean ± SD 5-HIAA (mg/L), mean ± SD
PD (n ¼ 82)
Control (n ¼ 64)
P
60.88 ± 10.77 29:53 3.35 ± 3.62
63.92 ± 10.60 28:36 e
0.09a 0.303b e
27.67 ± 13.60 2.66 ± 1.83 11.28 ± 6.30 13.85 ± 7.75 1.82 ± 0.76
e e e e e
e e e e e
201.14 ± 331.72 27.35 ± 3.14 11.13 ± 7.59 4.19 ± 1.60 5.78 ± 3.74 3.87 ± 2.25 11.68 ± 9.23 26.53 ± 15.07 96.48 ± 51.70
e 28.11 ± 2.85 0.86 ± 0.94 1.93 ± 0.68 0.31 ± 0.59 0.39 ± 0.88 0.64 ± 0.97 42.58 ± 16.99 186.75 ± 99.12
e 0.357c <0.001c <0.001c <0.001c <0.001c <0.001c <0.001c <0.001c
PD, Parkinson's disease; SD, Standard deviation; UPDRS, Unified Parkinson's disease rating scale; LEDD, levodopa-equivalent daily dose; MMSE, Mini-Mental State Examination; HAMD, Hamilton Depression Scale; FSS, Fatigue Severity Scale; PSQI, Pittsburgh Sleep Quality Index; VAS, Visual Analog Scale for Pain; SCOPA-AUT, Scale for Outcomes in PD for Autonomic Symptoms; 5-HT, serotonin; 5-HIAA, 5hydroxyindolacetic acid. a Student's t-test. b Chi-square test. c ManneWhitney test.
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Q. Tong et al. / Parkinsonism and Related Disorders xxx (2015) 1e6
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3. Results 3.1. Demographic and clinical characteristics of PD patients and controls
Fig. 1. Plasma levels of 5-HT and 5-HIAA in PD patients and controls. PD patients presented lower plasma levels of 5-HT and 5-HIAA than control individuals. ***P < 0.001; PD, Parkinson's disease; 5-HT, serotonin; 5-HIAA, 5-hydroxyindoleacetic acid.
data, and independent samples t-test or ManneWhitney U test for continuous variables as appropriate. Spearman's rank correlation analyses were conducted to evaluate the correlation between clinical variables and plasma levels of 5-HT and 5-HIAA. In order to further examine the impact of serotonin markers on NMS in PD, the multiple linear regression analysis was subsequently performed. Clinical and demographic features likely to affect NMS (gender, age, disease duration, UPDRS III score, H&Y stage and LEDD), as well as plasma levels of 5-HT and 5-HIAA were entered as independent variables in each regression model using respective NMS score as the dependent variable. P values less than 0.05 were considered statistically significant.
Demographic and clinical characteristics of all subjects are shown in Table 1. Eighty-two PD patients and sixty-four age- and gender-matched controls were recruited in this study. We found that NMS occurred frequently in Chinese PD patients: depression (48 patients), fatigue (43 patients), sleep disturbance (40 patients), pain (56 patients), autonomic dysfunction (62 patients). In the PD group, disease duration was significantly associated with HAMD score (r ¼ 0.272, P ¼ 0.013), FSS score (r ¼ 0.257, P ¼ 0.020) and VAS score (r ¼ 0.357, P ¼ 0.001). The severity and stage of PD, as assessed by UPDRS III score and H&Y stage were positively correlated with HAMD score (r ¼ 0.302, P ¼ 0.006; r ¼ 0.298, P ¼ 0.007, respectively). Moreover, LEDD was markedly associated with VAS score (r ¼ 0.391, P < 0.001) and SCOPA-AUT score (r ¼ 0.40, P < 0.001), while weakly correlated with PSQI score (r ¼ 0.272, P ¼ 0.014). 3.2. Plasma levels of 5-HT and 5-HIAA PD patients presented significantly lower plasma levels of 5-HT and 5-HIAA compared to control individuals (Table 1, Fig. 1). Within the PD group, as illustrated in Fig. 2, the decreased levels of plasma 5-HT and 5-HIAA were correlated with the severity of depression as evaluated by the HAMD score (r ¼ 0.447, P < 0.001; r ¼ 0.407, P < 0.001, respectively). Similarly, plasma levels of 5-HT and 5-HIAA were also negatively associated with VAS score (r ¼ 0.485, P < 0.001; r ¼ 0.416, P < 0.001, respectively). Nevertheless, we failed to observe any significant correlations between reduced levels of plasma 5-HT and 5-HIAA with the disease duration and
Fig. 2. Correlations between plasma levels of 5-HT and 5-HIAA and NMS in PD patients. (a): Plasma 5-HT level significantly correlated with HAMD score (r ¼ 0.447, r2 ¼ 0.207, P < 0.001); (b): Plasma 5-HIAA level significantly correlated with HAMD score (r ¼ 0.407, r2 ¼ 0.214, P < 0.001); (c): Plasma 5-HT level significantly correlated with VAS score (r ¼ 0.485, r2 ¼ 0.260, P < 0.001); (d): Plasma 5-HIAA level significantly correlated with VAS score (r ¼ 0.416, r2 ¼ 0.206, P < 0.001).
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Table 2 Multiple linear regression analysis with HAMD score as the dependent variable. Variables
Unstandardized coefficients (B)
SE
Standardized coefficients(b)
P
Gender Age Disease duration UPDRS III Hoehn and Yahr stage LEDD 5-HT 5-HIAA
3.844 0.028 0.352 0.236 0.058 0.003 0.143 0.044
1.454 0.067 0.244 0.123 1.333 0.003 0.054 0.015
0.244 0.040 0.168 0.244 0.006 0.148 0.284 0.300
0.010* 0.671 0.153 0.059 0.965 0.201 0.010* 0.006*
Adjusted R2
0.341(P < 0.001)* HAMD, Hamilton Depression Scale; SE, standard error of B; UPDRS, Unified Parkinson's disease rating scale; LEDD, levodopa-equivalent daily dose; 5-HT, serotonin; 5-HIAA, 5hydroxyindoleacetic acid; *P < 0.05.
staging, the UPDRS score and its subtests, and the antiparkinsonian medication intake, as well as other NMS assessed in this study, including fatigue, sleep disorders and autonomic dysfunction. In the control group, no significant correlations were found between plasma 5-HT and 5-HIAA concentrations and NMS. 3.3. Multiple linear regression analysis A multiple linear regression analysis with HAMD score as the dependent variable was carried out to evaluate the contribution of the factors including gender, age, disease duration, UPDRS III score, H&Y stage and LEDD, as well as plasma levels of 5-HT and 5-HIAA in HAMD (Table 2). The regression model was significant (P < 0.001) and accounted for 34.1% of variance in HAMD score. Moreover, plasma levels of 5-HT (b ¼ 0.284, P ¼ 0.010) and 5-HIAA (b ¼ 0.300, P ¼ 0.006), and gender (b ¼ 0.244, P ¼ 0.010) were found to be significantly correlated with HAMD score. Although the levels of plasma 5-HT and 5-HIAA negatively correlated with VAS score, as previously noted, the multiple regression analysis with VAS score as the dependent variable was not performed due to its non-normality even after several attempts at data transformation. 4. Discussion Over the last decade, accumulating evidence from postmortem, biochemical and neuroimaging studies suggests that serotonergic dysfunction, as measured in brain tissue and CSF, is involved in motor and nonmotor symptoms of PD, including depression, fatigue, pain, sleep and autonomic impairment [2,10,11,16]. However, less systematic work has been conducted to address the potential roles of 5-HT and 5-HIAA in PD by measuring their plasma levels. Emerging data from animal models and human studies have shown that concentrations of both 5-HT and 5-HIAA in plasma were strongly correlated with those in the brain tissue and CSF samples [17e19], suggesting that the plasma serotonin markers might partially reflect the serotonergic activity in the brain. Moreover, the procedure for collecting plasma samples is less invasive and much safer for patients compared to the CSF samples. Based on these data, we sought to determine levels of plasma 5-HT and 5-HIAA in PD patients, and investigate their associations with NMS. In this study, we observed that both plasma 5-HT and 5-HIAA levels were significantly decreased in PD patients, suggesting the presence of serotonergic dysfunction in PD. In line with our findings, previous studies have also shown decreased concentrations of 5-HT and 5-HIAA in brain and CSF of PD patients [6,9]. Moreover, striatal levels of the crucial serotonin markers, including 5-HT and 5-HIAA, as well as their transporter protein and synthesizing enzyme protein, were reduced in postmortem studies, providing compelling evidence for the involvement of serotonergic dysfunction in PD [1,20]. Consistently, several in vivo imaging studies have
confirmed some of these observations by showing a marked decrease of 5-HT transporter binding in the striatum of PD patients [7,8]. Taken together, these data support a crucial role of serotonergic dysfunction in the pathophysiology of PD. However, Engelborghs et al. found the CSF level of 5-HT was significantly higher in PD patients [21]. Additionally, a previous positron emission tomography (PET) study performed in nine patients with early PD has indicated that 5-HT transporter binding was not reduced in patients compared to healthy controls [22]. These variable findings might be attributable to differences in sample characteristics, different neurochemical analysis techniques, and differences in antiparkinsonian medications. Although the link between dopaminergic treatment and serotonergic dysfunction has yet to be discerned, several studies have suggested that chronic exposure to dopaminergic therapy might affect the serotonergic system in PD. For example, Chia et al. found a significant reduction of plasma 5-HT in PD patients receiving levodopa (L-DOPA) treatment [23]. Also, data from parkinsonian rats have suggested that chronic L-DOPA therapy could dramatically reduced striatal levels of 5-HT and 5HIAA [24]. These results were supported by an in vivo neurochemical study showing markedly reduced basal 5-HT release and metabolism in rat models of PD treated with a therapeutic dose of LDOPA [25]. However, in this study, we did not find any correlations between LEDD and plasma levels of 5-HT and 5-HIAA. A possible explanation for this discrepancy might be that all patients in the present study, in line with previous studies concerning serotonergic neurotransmission in PD [9,11,26], had their anti-parkinsonian medications stopped for 3 days before plasma sampling. To our knowledge, this is the first study to investigate the relationship between plasma levels of serotonin markers and NMS in PD patients. Although the pathophysiological mechanisms underlying NMS are complex and remain to be determined, converging evidence supports the concept that serotonergic dysfunction might be implicated in the occurrence of NMS. Based on Braak's hypothesis [5], serotonergic system that was thought to modulate several physiologic functions, such as emotion, sleep and wakefulness, and cognition, was affected in the neurodegenerative process of PD, suggesting a potential role of serotonergic pathway in NMS of PD. Mayeux et al. found the decreased CSF level of 5-HIAA was related to the presence of depression in PD patients [27]. Furthermore, a PET study has shown a significant reduction of 5-HT tracer uptake in the limbic system in depressed PD patients versus nondepressed patients [28]. In agreement with these observations, our current findings demonstrated negative associations between plasma levels of serotonin markers and the severity of depression in PD patients. However, previous studies have suggested that clinical characteristics such as gender, age, disease duration, and the disease severity and stage may be associated with depression in PD [29,30]. As such, in this study, we tried to keep all these confounding factors to a minimum by performing multiple linear regressions. After controlling for covariates, the decreased plasma
Please cite this article in press as: Q. Tong, et al., Reduced plasma serotonin and 5-hydroxyindoleacetic acid levels in Parkinson's disease are associated with nonmotor symptoms, Parkinsonism and Related Disorders (2015), http://dx.doi.org/10.1016/j.parkreldis.2015.05.016
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levels of 5-HT and 5-HIAA remained significantly associated with the severity of depression, which suggests that the altered serotonergic function might contribute to the genesis of depression in PD, and plasma levels of 5-HT and 5-HIAA may be considered as peripheral markers for depression of PD. Additionally, the findings from this study may provide evidence for the use of selective serotonin reuptake inhibitors (SSRI) as antidepressants in PD. In support, a placebo-controlled study has demonstrated that paroxetine significantly improved depressive symptoms in PD patients, which showed marked reductions of HAMD scores with drugs [31]. Nevertheless, these results should be taken cautiously as significant treatment effects of paroxetine were not observed in another placebo-controlled clinical trial [32]. Thus, further research aimed at evaluating the efficacy of serotonergic agents in depression of PD, with large sample size and long-duration, is warranted. However, there are inconsistent findings for the involvement of serotonergic dysfunction in the pathophysiology of depression in PD patients. For example, Olivola et al. found the reduced level of CSF 5-HT in PD patients was not associated with the presence of depression [9]. Additionally, a PET imaging study observed increased 5-HT transporter binding in depressed PD patients compared to healthy controls [33]. These discrepancies between studies may be due to the differences in sample size, the different methods of biological analysis and inclusion criteria. Compared with prior work, we recruited a relatively larger sample size of consecutive PD patients, of all disease duration and stage, and assessed the concentrations of serotonin markers in plasma samples by using a more sensitive and stable measurement. It is noteworthy that our study also observed the link between plasma levels of serotonin markers and the severity of pain in PD patients. In line with our findings, data from an animal study have revealed decreased 5-HT content of dialysates in contralateral ventrobasal thalamus and raphe magnus nucleus in rat models of neuropathic pain [34]. Recently, a clinical and neuroimaging study found that apomorphine has no influence on pain thresholds or pain-induced cerebral activity in PD patients as compared with placebo, providing evidence for the involvement of other nondopaminergic systems such as norepinephrine and serotonin in pain in PD [35]. Actually, the role of serotonergic system in pain modulation in PD has been confirmed by a study showing duloxetine, the selective 5-HT reuptake inhibitor, seems to be beneficial for the improvement of pain in PD patients [36]. Despite the fact that VAS score in our study was non-normally distributed and limited the following statistical analysis, our findings indicate that serotonergic dysfunction might contribute to the pathogenesis of pain in PD. Of note, we are aware that there are some limitations in this study. First, we performed a peripheral assessment of serotonergic dysfunction by evaluating 5-HT and 5-HIAA in plasma samples; second, the sample size of our study was relative small for correlation analysis; third, our work was restricted to five types of NMS in PD patients, and consequently ignored other NMS such as cognitive impairments. Therefore, larger scale studies employing multiple approaches such as PET are still indispensable to elucidate the role of serotonergic dysfunction in PD, particularly with respect to NMS, as they may provide crucial evidence for the treatment. In conclusion, our study showed that PD patients exhibited lower plasma levels of 5-HT and 5-HIAA than control individuals, and the decreased plasma serotonin markers were correlated with the severity of depression and pain. Combined with the findings from previous work, our results suggest that serotonergic dysfunction may contribute to nonmotor symptoms in PD patients, and plasma levels of serotonin markers would be considered as peripheral markers for depression in PD.
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