Tremor is associated with PET measures of nigrostriatal dopamine function in MPTP-lesioned monkeys

Monitoring of a progressive functional dopaminergic deficit in the A53T-AAV synuclein rats by combining 6-[18F]fluoro-L-m-tyrosine imaging and motor performances analysis

With the emergence of disease-modifying therapies for Parkinson's disease, reliable longitudinal markers are needed to quantify pathology and demonstrate disease progression. We developed the A53T-AAV rat model of synucleinopathy by combining longitudinal measures over 12 weeks. We first characterized the progression of the motor and dopaminergic deficits. Then, we monitored the disease progression using the [18F]FMT Positron Emission Tomography (PET) radiotracer. The nigral injection of A53T-AAV led to an increase in phosphorylated α-synuclein on S129, a progressive accumulation of α-synuclein aggregates, and a decrease of dopaminergic function associated with a deterioration of motor activity. The longitudinal monitoring of A53T-AAV rats with [18F]FMT PET showed a progressive reduction of the Kc outcome parameter in the caudate putamen from the lesioned side. Interestingly, the progressive reduction in the [18F]FMT PET signal correlated with defects in the stepping test. In conclusion, we established a progressive rat model of α-synuclein pathology which monitors the deficit longitudinally using both the [18F]FMT PET tracer and behavioral parameters, 2 features that have strong relevance for translational approaches.

Rest tremor correlates with reduced contralateral striatal dopamine transporter binding in Parkinson's disease

INTRODUCTION

In vivo dopamine transporter imaging is a useful tool for distinguishing nigrostriatal pathologies (e.g. Parkinson's disease) from other causes of tremor. However, while many of the motoric features of Parkinson's disease (e.g. bradykinesia, rigidity, hypomimia) correlate well with reduced striatal dopamine transporter binding, the same relationship has not been demonstrated for tremor. We investigated the relationship between striatal dopamine transporter binding and quantitative measures of tremor.

METHODS

23 participants with Parkinson's disease underwent standardised clinical assessment including structured, videotaped clinical examination, tremor neurophysiology study of both upper limbs using accelerometry and surface EMG, and Technitium-99 m TRODAT-1 brain SPECT imaging. Normalised striatal uptake values were calculated. Tremor EMG and accelerometry time series were processed with Fourier transformation to identify peak tremor power within a window of 3-10Hz and to calculate the tremor stability index (TSI).

RESULTS

Spearman correlation analyses revealed an association between tremor power and contralaterally reduced striatal uptake in a number of recording conditions. This association was strongest for rest tremor, followed by postural tremor, with the weakest association observed for kinetic tremor. Lower TSI was also associated with lower contralateral striatal uptake in a number of rest and postural conditions.

CONCLUSION

These data suggest a relationship between Parkinsonian rest tremor and contralateral reduction in striatal dopamine binding. Use of quantitative neurophysiology techniques may allow the demonstration of clinico-pathophysiological relationships in tremor that have remained occult to previous studies.

6-OHDA injections into A8-A9 dopaminergic neurons modelling early stages of Parkinson's disease increase the harmaline-induced tremor in rats

The aim of the present study was to examine the influence of a unilateral 6-hydroxydopamine (6-OHDA)-induced partial lesion of both the substantia nigra pars compacta (SNc, A9) and retrorubral field (RRF, A8) on the tremor evoked by harmaline. 6-OHDA (8μg/2μl) was injected unilaterally into the region of the posterior part of the SNc and RRF. Harmaline was administered in a dose of 7.5mg/kg ip on the eighth day after the operation and tremor of forelimbs, head and trunk was measured. We found that the lesion increased intensity of the tremor induced by harmaline but did not influence its character. Stereological examination of the lesion extent revealed losses of dopaminergic (tyrosine hydroxylase-immunoreactive) neurons in the anterior (30%) and posterior (72%) SNc, as well as in RRF (72% on the average). Levels of dopamine and all its metabolites, as well as noradrenaline concentrations, were ipsilaterally moderately decreased in the caudate-putamen in the lesioned animals, however, dopamine and DOPAC in the anterior cerebellum were increased. In the caudate-putamen, the ipsi/contra ratio of dopamine level correlated negatively, while that of dopamine turnover positively with the tremor intensity. However, in the anterior cerebellum an inverse relationship was found. Moreover, this symptom correlated positively with the serotonin level and negatively with the 5-HIAA/serotonin ratio on the contralateral side of the posterior cerebellum. The present results seem to indicate that the modulation of dopaminergic and serotonergic transmissions by the lesion modelling early stages of Parkinson's disease may influence tremor triggered in the cerebellum.

Testing the contributions of striatal dopamine loss to the genesis of parkinsonian signs

The diverse and independently-varying signs of Parkinson's disease (PD) are often attributed to one simple mechanism: degeneration of the dopaminergic innervation of the posterolateral striatum. However, growing recognition of the dopamine (DA) loss and other pathology in extra-striatal brain regions has led to uncertainty whether loss of DA in the striatum is sufficient to cause parkinsonian signs. We tested this hypothesis by infusing cis-flupenthixol (cis-flu; a broad-spectrum D1/D2 receptor antagonist) into different regions of the macaque putamen (3 hemispheres of 2 monkeys) while the animal performed a visually-cued choice reaction time task in which visual cues indicated the arm to reach with and the peripheral target to contact to obtain food reward. Following reward delivery, the animal was required to self-initiate release of the peripheral target and return of the chosen hand to its home position (i.e., without the benefit of external sensory cues or immediate rewards). Infusions of cis-flu at 15 of 26 sites induced prolongations of reaction time (9 of 15 cases), movement duration (6 cases), and/or dwell time of the hand at the peripheral target (8 cases). Dwell times were affected more severely (+95%) than visually-triggered reaction times or movement durations (+25% and +15%, respectively). Specifically, the animal's hand often 'froze' at the peripheral target for up to 25-s, similar to the akinetic freezing episodes observed in PD patients. Across injections, slowing of self-initiation did not correlate in severity with prolongations of visually-triggered reaction time or movement duration, although the latter two were correlated with each other. Episodes of slowed self-initiation appeared primarily in the arm contralateral to the injected hemisphere and were not associated with increased muscle co-contraction or global alterations in behavioral state (i.e., inattention or reduced motivation), consistent with the idea that these episodes reflected a fundamental impairment of movement initiation. We found no evidence for an anatomic topography within the putamen for the effects elicited. We conclude that acute focal blockade of DA transmission in the putamen is sufficient to induce marked akinesia-like impairments. Furthermore, different classes of impairments can be induced independently, suggesting that specific parkinsonian signs have unique pathophysiologic substrates.

A within-subject comparison of 6-[18F]fluoro-m-tyrosine and 6-[18F]fluoro-L-dopa in Parkinson's disease

Progression of Parkinson's disease symptoms is imperfectly correlated with positron emission tomography biomarkers for dopamine biosynthetic pathways. The radiopharmaceutical 6-[(18) F]fluoro-m-tyrosine is not a substrate for catechol-O-methyltransferase and therefore has a more favorable uptake-to-background ratio than 6-[(18) F]fluoro-L-dopa. The objective of this study was to evaluate 6-[(18) F]fluoro-m-tyrosine relative to 6-[(18) F]fluoro-L-dopa with partial catechol-O-methyltransferase inhibition as a biomarker for clinical status in Parkinson's disease. Twelve patients with early-stage Parkinson's disease, off medication, underwent Unified Parkinson Disease Rating Scale scoring, brain magnetic resonance imaging, and 3-dimensional dynamic positron emission tomography using equivalent doses of 6-[(18) F]fluoro-m-tyrosine and 6-[(18) F]fluoro-L-dopa with tolcapone, a catechol-O-methyltransferase inhibitor. Images were realigned within subject, after which the tissue-derived uptake rate constant was generated for volumes of interest encompassing the caudate nucleus, putamen, and subregions of the putamen. We computed both bivariate (Pearson) and partial (covariate of age) correlations between clinical subscores and tissue-derived uptake rate constant. Tissue-derived uptake rate constant values were correlated between the radiopharmaceuticals (r = 0.8). Motor subscores were inversely correlated with the contralateral putamen 6-[(18) F]fluoro-m-tyrosine tissue-derived uptake rate constant (|r| > 0.72, P < .005) but not significantly with the 6-[(18) F]fluoro-L-dopa tissue-derived uptake rate constant. The uptake rate constants for both radiopharmaceuticals were also inversely correlated with activities of daily living subscores, but the magnitude of correlation coefficients was greater for 6-[(18) F]fluoro-m-tyrosine. In this design, 6-[(18) F]fluoro-m-tyrosine uptake better reflected clinical status than did 6-[(18) F]fluoro-L-dopa uptake. We attribute this finding to 6-[(18) F]fluoro-m-tyrosine's higher affinity for the target, L-aromatic amino acid decarboxylase, and the absence of other major determinants of the uptake rate constant. These results also imply that L-aromatic amino acid decarboxylase activity is a major determinant of clinical status.

Functional effects of AAV2-GDNF on the dopaminergic nigrostriatal pathway in parkinsonian rhesus monkeys

We investigated the safety and neuroregenerative potential of an adeno-associated virus (AAV2) containing human glial cell line-derived neurotrophic factor (GDNF) in an MPTP primate model of Parkinson's disease. Dopaminergic function was evaluated by positron emission tomography with 6-[(18)F]fluoro-l-m-tyrosine (FMT) before and after AAV2-GDNF or phosphate-buffered saline infusion bilaterally into the putamen. FMT uptake was significantly increased bilaterally in the putamen of AAV2-GDNF but not phosphate-buffered saline-treated animals 6 months after infusion, indicating increased dopaminergic activity in the nigrostriatal pathways. AAV2-GDNF-treated animals also showed clinical improvement without adverse effects. These findings are consistent with our previous report in aged nonhuman primates that showed evidence of enhanced use of striatal dopamine and dopaminergic nigrostriatal innervation. Clinical improvement and evidence of functional recovery in the nigrostriatal pathway, and the absence of adverse effects, support the safety of this approach for the delivery of GDNF over a 6-month period.

PET 6-[F]fluoro-L-m-tyrosine Studies of Dopaminergic Function in Human and Nonhuman Primates

Although positron emission tomography (PET) and the aromatic L-amino acid decarboxylase (AADC) tracer 6-[¹⁸F]fluoro-L-m-tyrosine (FMT) has been used to assess the integrity of the presynaptic dopamine system in the brain, relatively little has been published in terms of brain FMT uptake values especially for normal human subjects. Twelve normal volunteer subjects were scanned using PET and FMT to determine the range of normal striatal uptake values using Patlak graphical analysis. For comparison, seven adult rhesus monkeys were studied and the data analyzed in the same way. A subset of monkeys that were treated with a unilateral intracarotid artery infusion of the dopamine neurotoxin MPTP showed an 87% decrease in striatal FMT uptake. These findings support the use of PET and FMT to image AADC distribution in both normal and diseased brains using Patlak graphical analysis and tissue input functions.

Animal Models of Neurodegenerative Disease: Insights from In vivo Imaging Studies

Animal models have been used extensively to understand the etiology and pathophysiology of human neurodegenerative diseases, and are an essential component in the development of therapeutic interventions for these disorders. In recent years, technical advances in imaging modalities such as positron emission tomography (PET) and magnetic resonance imaging (MRI) have allowed the use of these techniques for the evaluation of functional, neurochemical, and anatomical changes in the brains of animals. Combining animal models of neurodegenerative disorders with neuroimaging provides a powerful tool to follow the disease process, to examine compensatory mechanisms, and to investigate the effects of potential treatments preclinically to derive knowledge that will ultimately inform our clinical decisions. This article reviews the literature on the use of PET and MRI in animal models of Parkinson's disease, Huntington's disease, and Alzheimer's disease, and evaluates the strengths and limitations of brain imaging in animal models of neurodegenerative diseases.

Swim-test as a function of motor impairment in MPTP model of Parkinson's disease: A comparative study in two mouse strains

Parkinson's disease (PD) is a common neurodegenerative disease that exhibits motor dysfunctions, such as tremor, akinesia and rigidity. In the present study, we investigated whether swim-test could be used as one of the behavioural monitoring techniques to study motor disability in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced parkinsonism in two mouse strains, Balb/c and C57BL/6. Mice were treated with different doses of MPTP (10, 20 and 30 mg/kg, twice, 16 h apart), and were subjected to swim-test on the third day of the first MPTP injection. MPTP-induced tremor was monitored at 30 min, and akinesia and rigidity developed were studied 3 h after the second MPTP treatment. While tremor and akinesia produced were dose-dependent and the intensity of tremor was comparable in the two strains of mice studied, the latter response in C57BL/6 was significantly lesser than that observed in Balb/c. Rigidity exhibited in Balb/c mice were dose-dependent, but not in C57BL/6. There was observed an inverse relationship between swim-score and the doses of MPTP in both the strains. MPTP caused a significant and dose-dependent reduction in striatal dopamine level in both the strains of mice, when assayed on the fourth day employing an HPLC with electrochemical detector. A significant positive correlation existed (r = 0.94 for Balb/c and r = 0.82 for C57BL/6) for the striatal dopamine-depletion and the swim-score in the MPTP-treated mice. While swim deficit and striatal dopamine loss were long lasting (till the third week) in C57BL/6, in Balb/c mice the motor deficit showed recovery by the second week. In these animals, a significant attenuation in striatal dopamine loss was observed by the third week. These results indicate that swim ability is directly proportional to striatal dopamine content, and suggest that swim-test could be used as a major technique to monitor motor dysfunction in experimental animals.

Caloric restriction increases neurotrophic factor levels and attenuates neurochemical and behavioral deficits in a primate model of Parkinson's disease

We report that a low-calorie diet can lessen the severity of neurochemical deficits and motor dysfunction in a primate model of Parkinson's disease. Adult male rhesus monkeys were maintained for 6 months on a reduced-calorie diet [30% caloric restriction (CR)] or an ad libitum control diet after which they were subjected to treatment with a neurotoxin to produce a hemiparkinson condition. After neurotoxin treatment, CR monkeys exhibited significantly higher levels of locomotor activity compared with control monkeys as well as higher levels of dopamine (DA) and DA metabolites in the striatal region. Increased survival of DA neurons in the substantia nigra and improved manual dexterity were noted but did not reach statistical significance. Levels of glial cell line-derived neurotrophic factor, which is known to promote the survival of DA neurons, were increased significantly in the caudate nucleus of CR monkeys, suggesting a role for glial cell line-derived neurotrophic factor in the anti-Parkinson's disease effect of the low-calorie diet.

Comparison of two methods for the analysis of [18F]6-fluoro-L-m-tyrosine PET data

PET and [18F]fluoro-L-m-tyrosine (FMT) have been used to quantify presynaptic striatal dopamine (DA) function in Parkinson disease (PD) and in primate models of PD. While dynamic imaging and a metabolite-corrected blood input function can be used to determine striatal FMT uptake rate constants (Ki), a simpler analytic approach using shorter imaging times is desirable for clinical studies. We compared the utility of using striatal Ki values versus striatal count ratios in two groups of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated monkeys. Striatal DA content was also measured in one of the groups to evaluate the relationship between the PET measures and an independent measure of striatal dopamine. Striatal Ki values were significantly correlated with striatal count ratios using the cerebellum as the denominator. Both Ki values and ratios were also correlated with striatal DA content. In addition, putamen-cerebellum ratios and putamen Ki values showed similar separation between baseline and post-MPTP values. These findings suggest that a simple ratio approach to analyzing FMT PET data may be a useful alternative to a kinetic approach especially for clinical applications.

Evaluation of animal models of Parkinson's disease for neuroprotective strategies

Parkinson's disease (PD) is a common neurodegenerative disorder characterized by the progressive loss of dopaminergic nigral neurons and striatal dopamine. Despite the advances of modern therapy to treat the symptoms of PD, most of the patients will eventually experience debilitating disability. The need for neuroprotective strategies that will slow or stop the progression of the disease is clear. The progress in the understanding of the cause and pathogenesis of PD is providing clues for the development of disease-modifying strategies. In that regard, animal models of PD and non-human primate models in particular, are essential for the preclinical evaluation and testing of candidate therapies. However, the diversity of models and different outcome measures used by investigators make it challenging to compare results between neuroprotective agents. In this review we will discuss methods for the selection, development and assessment of animal models of PD, the role of non-human primates and the concept of "multiple models/multiple endpoints" to predict the success in the clinic of neuroprotective strategies.

Anomalies of asymmetry of clinical signs in parkinsonism

Parkinson's disease (PD) is characterized by a minimum of two of three features: tremor, rigidity, and bradykinesia. Asymmetry of these features is often considered to support a diagnosis of PD in contrast to other parkinsonian syndromes. All major manifestations of PD are often more pronounced on the side first manifesting features of PD. Significant dissociation of features on the contralateral side, along with other variants of presentation involving the contralateral side, are rarely observed. To determine the frequency and significance of unusual asymmetry in parkinsonism, we retrospectively examined 613 patients clinically diagnosed as idiopathic PD for presence of unusual asymmetries of clinical features. Three groups of patients with unusual asymmetrical clinical findings were identified. Group 1 comprised 10 patients followed for an average of 6 years presenting with rest tremor most prominent in one lower limb and contralateral upper limb. Group 2 comprised 24 patients followed for an average of 5.5 years with action tremor most prominent on the side contralateral to the side of most prominent rest tremor. Group 3 comprised 33 patients followed for an average of 10 years who had parkinsonian signs of greatest severity on one side but subsequently, over an average of 5.4 years, became gradually more prominent on the opposite side. In Group 3, 15 of 33 patients (45%) demonstrated evolution to a rigid form of parkinsonism with disappearance of rest tremor over an average of 7.1 years after presentation. A small percentage (11%) of Parkinson's patients in our clinic demonstrated anomalous asymmetrical clinical findings, which indicates that (1) the disease process may begin in different topographic sites on each side; (2) rest tremor and action tremor may have different anatomical bases; (3) the disease process may progress at different rates on different sides; and (4) tremor becomes less pronounced with progression of disease in some patients with Parkinsonism.