Imaging Non-Dopaminergic Function in Parkinson’s Disease

Serotonin as a biomarker of toxin-induced Parkinsonism

BACKGROUND

Loss of dopaminergic neurons underlies the motor symptoms of Parkinson's disease (PD). However stereotypical PD symptoms only manifest after approximately 80% of dopamine neurons have died making dopamine-related motor phenotypes unreliable markers of the earlier stages of the disease. There are other non-motor symptoms, such as depression, that may present decades before motor symptoms.

METHODS

Because serotonin is implicated in depression, here we use niche, fast electrochemistry paired with mathematical modelling and machine learning to, for the first time, robustly evaluate serotonin neurochemistry in vivo in real time in a toxicological model of Parkinsonism, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP).

RESULTS

Mice treated with acute MPTP had lower concentrations of in vivo, evoked and ambient serotonin in the hippocampus, consistent with the clinical comorbidity of depression with PD. These mice did not chemically respond to SSRI, as strongly as control animals did, following the clinical literature showing that antidepressant success during PD is highly variable. Following L-DOPA administration, using a novel machine learning analysis tool, we observed a dynamic shift from evoked serotonin release in the hippocampus to dopamine release. We hypothesize that this finding shows, in real time, that serotonergic neurons uptake L-DOPA and produce dopamine at the expense of serotonin, supporting the significant clinical correlation between L-DOPA and depression. Finally, we found that this post L-DOPA dopamine release was less regulated, staying in the synapse for longer. This finding is perhaps due to lack of autoreceptor control and may provide a ground from which to study L-DOPA induced dyskinesia.

CONCLUSIONS

These results validate key prior hypotheses about the roles of serotonin during PD and open an avenue to study to potentially improve therapeutics for levodopa-induced dyskinesia and depression.

The Association of Olfactory Dysfunction With Depression, Cognition, and Disease Severity in Parkinson's Disease

Background: Non-motor subtypes of Parkinson's disease (PD) include the limbic, cognitive, and brainstem phenotype, which may have different pathological pathways with olfaction. In this work, we aim to clarify the association between olfactory dysfunction, depression, cognition, and disease severity in PD.

Methods: A total of 105 PD subjects were included and divided into anosmia and non-anosmic groups, using the University of Pennsylvania Smell Identification Test (UPSIT). All patients were evaluated with the movement disorder society unified Parkinson's disease rating scale (MDS-UPDRS), the Beck depression inventory (BDI)-II, and the Montreal cognitive assessment (MoCA).

Results: The BDI-II and UPSIT scores had a trend of reverse correlation without statistical significance (β-coefficient −0.12, p = 0.232). However, the odds ratio (OR) in anosmia was 2.74 (95% CI 1.01–7.46) for depression and 2.58 (95% CI 1.06–6.29) for cognitive impairment. For the MDS-UPDRS total and Part 3 score, the anosmia had a β-coefficient of 12.26 (95% CI 5.69–18.82) and 8.07 (95% CI 3.46–12.67), respectively. Neither depression nor cognitive impairment is associated with motor symptoms.

Conclusion: More severe olfactory dysfunction in PD is associated with cognitive impairment and greater disease severity. Depression in PD may involve complex pathways, causing relatively weak association with olfactory dysfunction.

Parkinson's disease--the debate on the clinical phenomenology, aetiology, pathology and pathogenesis

The definition of Parkinson’s disease (PD) is changing with the expansion of clinical phenomenology and improved understanding of environmental and genetic influences that impact on the pathogenesis of the disease at the cellular and molecular level. This had led to debate and discussion with as yet, no general acceptance of the direction that change should take either at the level of diagnosis or of what should and should not be sheltered under an umbrella of PD. This article is one contribution to this on-going discussion. There are two different themes running through the article - widening the definition of PD/LBD/synucleinopathies and the heterogeneity that exists within PD itself from a clinical, pathological and genetic per-spective. The conclusion reached is that in the future, further diagnostic categories will need to be recognized. These are likely to include - Parkinson’s syndrome, Parkinson’s syndrome likely to be Lewy body PD, clinical PD (defined by QSBB criteria), Lewy body disease (PD, LBD, REM SBD) and synucleinopathies (including LBD, MSA).

Antidepressants for depression in Parkinson's disease: systematic review and meta-analysis

Depression is common in Parkinson's disease (PD) and is associated with several poor outcomes. However the literature regarding treatment with antidepressants in this population is controversial. The aim of this paper was to systematically review all randomized controlled trials that studied the efficacy of antidepressants for depression in PD (dPD). Studies were retrieved from PubMed (1966-July 2012), Cochrane Library (-July 2012, issue 7), Embase (1980-July 2012), PsycINFO (1980-July 2012), Lilacs (1982-July 2012), secondary references, clinical trials registries and a thesis database. Only double-blind, randomized controlled trials in which an antidepressant was given as the main treatment and compared with placebo and/or another antidepressant were included. Out of the 1438 studies retrieved, only six could be included. Taking into account the five placebo-controlled trials, the overall risk ratio (RR) for response was 1.36 (0.98, 1.87), indicating no statistically significant superiority of antidepressants over placebo. However, in the sensitivity analysis, the RR for response was 1.41 (1.01, 1.96) and 1.48 (1.05, 2.10) after exclusion of one study with questionable results, and when only studies with low risk of bias were considered, respectively. No specific antidepressant class was superior to placebo. In general antidepressant medications were well tolerated. The results suggest antidepressants may be efficacious in the treatment of dPD. However, the results were unstable. In fact, the small number of trials and methodological drawbacks preclude definitive conclusions about their efficacy and tolerability in dPD.

Cognitive slowing in Parkinson's disease is related to frontostriatal dopaminergic dysfunction

BACKGROUND

Frontostriatal and cognitive dysfunctions in Parkinson's disease (PD) are hypothesized to be linked predominately to dopaminergic dysfunction within neural networks linking dorsal striatum to dorsolateral prefrontal cortex.

METHODS

The authors evaluated the relationship between frontostriatal dopaminergic function and cognitive performance, especially cognitive processing speed by performing [(18)F]fluorodopa PET and computerized tests of automatic and controlled cognitive processing speed (CogniSpeed) in 23 newly diagnosed and unmedicated PD patients and 14 controls.

RESULTS

PD patients were slower than the controls in all the CogniSpeed measures studied. The Fdopa uptake in caudate nucleus correlated negatively with slowing on all the tests. Slower performance in relatively automatic processes measured by choice reaction tasks as well as in more controlled processes measured by a calculation task was related to reduced Fdopa uptake in the anterior cingulate gyrus. The reduced dopaminergic function in the thalamus was associated with the slower performance in the subtraction test.

CONCLUSION

Our study indicates that dopaminergic dysfunction within neural networks linking striatum to prefrontal cortex is involved in the slowing of both automatic and controlled cognitive processing in PD patients.

PET imaging of serotonin transporters with 4-[18F]-ADAM in a Parkinsonian rat model

This study was undertaken to address the effects of fetal mesencephalic tissue transplantation on the serotonin system in a rat model of Parkinson's disease (PD) while also investigating the usefulness of 4-[18F]-ADAM (a serotonin transporter imaging agent) coupled with micro-PET for imaging serotonin transporters (SERTs). A PD model was induced by unilateral injection of 6-hydroxydopamine (6-OHDA) into the right medial forebrain bundle of the nigrostriatal pathway, while cell transplantation was performed via intrastriatal injection of mesencephalic brain tissue dissected from embryonic (E14) rats. The 4-[18F]-ADAM/micro-PET scanning was performed following both 6-OHDA lesioning and transplantation. Immunohistochemistry (IHC) studies were also performed following the final PET scan, and the results were compared to show a 17-43% decrease in the specific uptake ratio (SUR) and a 23-52% decrease in serotonin transporter immunoreactivity (SERT-ir) within various brain regions on the lesioned side. The number of methamphetamine-induced rotations also decreased significantly at the 4th week postgraft. In addition, striatal SUR and the SERT-ir levels were restored to 77% and 83% 5 weeks postgraft. These results suggest that Parkinson's disease also affects the serotonergic system, while both the dopaminergic and serotonergic systems can be partially restored in a rat model of PD after E14 mesencephalic tissue transplantation. In addition, we have also determined that 4-[18F]-ADAM/micro-PET can be used to detect serotonergic neuron loss, monitor the progress of Parkinson's disease, and oversee the effectiveness of therapy.

Mathematical insights into the effects of levodopa

Parkinson’s disease has been traditionally thought of as a dopaminergic disease in which cells of the substantia nigra pars compacta (SNc) die. However, accumulating evidence implies an important role for the serotonergic system in Parkinson’s disease in general and in physiological responses to levodopa therapy, the first line of treatment. We use a mathematical model to investigate the consequences of levodopa therapy on the serotonergic system and on the pulsatile release of dopamine (DA) from dopaminergic and serotonergic terminals in the striatum. Levodopa competes with tyrosine and tryptophan at the blood-brain barrier and is taken up by serotonin neurons in which it competes for aromatic amino acid decarboxylase. The DA produced competes with serotonin (5HT) for packaging into vesicles. We predict the time courses of LD, cytosolic DA, and vesicular DA in 5HT neurons during an LD dose. We predict the time courses of DA and 5HT release from 5HT cell bodies and 5HT terminals as well as the changes in 5HT firing rate due to lower 5HT release. We compute the time course of DA release in the striatum from both 5HT and DA neurons and show how the time course changes as more and more SNc cells die. This enables us to explain the shortening of the therapeutic time window for the efficacy of levodopa as Parkinson’s disease progresses. Finally, we study the effects 5HT1a and 5HT1b autoreceptor agonists and explain why they have a synergistic effect and why they lengthen the therapeutic time window for LD therapy. Our results are consistent with and help explain results in the experimental literature and provide new predictions that can be tested experimentally.

Parkinson's disease motor subtypes and mood

Parkinson's disease is heterogeneous, both in terms of motor symptoms and mood. Identifying associations between phenotypic variants of motor and mood subtypes may provide clues to understand mechanisms underlying mood disorder and symptoms in Parkinson's disease. A total of 513 patients were assessed using the Hospital Anxiety and Depression Scale, and separately classified into anxious, depressed, and anxious-depressed mood classes based on latent class analysis of a semistructured interview. Motor subtypes assessed related to age-of-onset, rate of progression, presence of motor fluctuations, lateralization of motor symptoms, tremor dominance, and the presence of postural instability and gait symptoms and falls. The directions of observed associations tended to support previous findings with the exception of lateralization of symptoms, for which there were no consistent or significant results. Regression models examining a range of motor subtypes together indicated increased risk of anxiety in patients with younger age-of-onset and motor fluctuations. In contrast, depression was most strongly related to axial motor symptoms. Different risk factors were observed for depressed patients with and without anxiety, suggesting heterogeneity within Parkinson's disease depression. Such association data may suggest possible underlying common risk factors for motor subtype and mood. Combined with convergent evidence from other sources, possible mechanisms may include cholinergic system damage and white matter changes contributing to non-anxious depression in Parkinson's disease, while situational factors related to threat and unpredictability may contribute to the exacerbation and maintenance of anxiety in susceptible individuals.

Imaging cognitive and behavioral symptoms in Parkinson's disease

Non-motor symptoms are a major and often unrecognized cause of morbidity of Parkinson's disease. In the past few years, imaging technology, such as functional MRI and PET, have provided a large bulk of information about the phenomena. Here, we provide an overview of those imaging studies that may help us understand the neuronal correlates associated with non-motor symptoms in Parkinson's disease, with a particular focus on cognitive and neuropsychiatric deficits.

Efficacy and acceptability of selective serotonin reuptake inhibitors for the treatment of depression in Parkinson's disease: a systematic review and meta-analysis of randomized controlled trials

Background

Selective serotonin reuptake inhibitors (SSRIs) are the most commonly prescribed antidepressants for the treatment of depression in patients with Parkinson's Disease (PD) but data on their efficacy are controversial.

Methods

We conducted a systematic review and meta-analysis of randomized controlled trials to investigate the efficacy and acceptability of SSRIs in the treatment of depression in PD.

Results

Ten studies were included. In the comparison between SSRIs and Placebo (n = 6 studies), the combined risk ratio (random effects) was 1.08 (95% confidence interval: 0.77 - 1.55, p = 0.67). In the comparison between SSRIs and Tricyclic Antidepressants (TCAs) (n = 3 studies) the combined risk ratio was 0.75 (0.39 - 1.42, p = 0.37). An acceptability analysis showed that SSRIs were generally well tolerated.

Conclusions

These results suggest that there is insufficient evidence to reject the null hypothesis of no differences in efficacy between SSRIs and placebo in the treatment of depression in PD. Due to the limited number of studies and the small sample sizes a type II error (false negative) cannot be excluded. The comparison between SSRIs and TCAs is based on only three studies and further trials with more pragmatic design are needed.

Does smoking reduce the risk of Parkinson's disease through stimulation of the ubiquitin-proteasome system?

Parkinson's disease is a neurodegenerative condition characterized by tremor, rigidity, bradykinesia, and postural instability. Smoking is an inverse risk factor for Parkinson's disease, although the mechanism for this apparent neuroprotection is not definitively established. Smoking consistently upregulates nicotinic acetylcholine receptor levels in various brain regions known to be involved in Parkinson's disease. The ubiquitin-proteasome system--the system that tags and removes unwanted, misfolded, or damaged proteins from cells--regulates nicotinic receptor levels. The ubiquitin-proteasome system has also been implicated in Parkinson's disease, with aberrant activity identified in both sporadic and familial forms of the disease. The involvement of the ubiquitin-proteasome system in nicotinic receptor regulation and Parkinson's disease pathology suggests a link between the two, which forms the basis of the present hypothesis. Specifically, this paper considers the hypothesis that smoking reduces the risk of Parkinson's disease through the upregulation of nicotinic cholinergic receptors in key brain regions involved in Parkinson's disease. This receptor upregulation is hypothesized to increase activity of the ubiquitin-proteasome system, which is believed to prevent neurodegeneration caused by the accumulation of misfolded or damaged proteins or other consequences of inadequate protein sequestration and/or degradation. This hypothesis is supported by evidence documenting the upregulation of nicotinic receptors in the brains of smokers, neuroprotective effects of nicotine, reduced activity of the ubiquitin-proteasome in Parkinson's disease, and increased activity of the ubiquitin-proteasome system in animals exposed to chronic nicotine. Additional research is needed to test several predictions of the hypothesis, including increased activity of the ubiquitin-proteasome system in key brain regions of smokers.

Assessment of the progression of Parkinson's disease: a metabolic network approach

BACKGROUND

Clinical research into Parkinson's disease has focused increasingly on the development of interventions that slow the neurodegeneration underlying this disorder. These investigations have stimulated interest in finding objective biomarkers that show changes in the rate of disease progression with treatment. Through radiotracer-based imaging of nigrostriatal dopaminergic function, a specific class of biomarkers to monitor the progression of Parkinson's disease has been identified, and these biomarkers were used in the clinical trials of drugs with the potential to modify the course of the disease. However, in some of these studies there was discordance between the imaging outcome measures and blinded clinical ratings of disease severity. Research is underway to identify and validate alternative ways to image brain metabolism, through which the efficacy of new therapies for Parkinson's disease and related disorders can be assessed.

RECENT DEVELOPMENTS

During recent years, spatial covariance analysis has been used with (18)F-fluorodeoxyglucose PET to detect abnormal patterns of brain metabolism in patients with neurodegenerative disorders. Rapid, automated, voxel-based algorithms have been used with metabolic imaging to quantify the activity of disease-specific networks. This approach has helped to characterise the unique metabolic patterns associated with the motor and cognitive features of Parkinson's disease. The results of several studies have shown correction of abnormal motor, but not cognitive, network activity by treatment with dopaminergic therapy and deep brain stimulation. The authors of a longitudinal imaging study of early-stage Parkinson's disease reported substantial differences in the development of these metabolic networks over a follow-up of 4 years. WHERE NEXT?: Developments in network imaging have provided the basis for several new applications of metabolic imaging in the study of Parkinson's disease. A washout study is currently underway to determine the long-duration effects of dopaminergic therapy on the network activity related to Parkinson's disease, which will be useful to plan future trials of disease-modifying drugs. Network approaches are also being applied to the study of atypical parkinsonian syndromes. The characterisation of specific patterns associated with atypical parkinsonian syndromes and classic Parkinson's disease will be the basis for a fully automated imaging-based procedure for early differential diagnosis. Efforts are underway to quantify the networks related to Parkinson's disease with less invasive imaging methods. Assessments of network activity with perfusion-weighted MRI show excellent concordance with measurements done with established radiotracer techniques. This approach will ultimately enable the assessment of abnormal network activity in people who are genetically at risk of Parkinson's disease.