A longitudinal study of motor performance and striatal [18F]fluorodopa uptake in Parkinson's disease

Evaluation of Intraperitoneal [18F]-FDOPA Administration for Micro-PET Imaging in Mice and Assessment of the Effect of Subchronic Ketamine Dosing on Dopamine Synthesis Capacity

Positron emission tomography (PET) using the radiotracer [¹⁸F]-FDOPA provides a tool for studying brain dopamine synthesis capacity in animals and humans. We have previously standardised a micro-PET methodology in mice by intravenously administering [¹⁸F]-FDOPA via jugular vein cannulation and assessment of striatal dopamine synthesis capacity, indexed as the influx rate constant K _i ^Mod of [¹⁸F]-FDOPA, using an extended graphical Patlak analysis with the cerebellum as a reference region. This enables a direct comparison between preclinical and clinical output values. However, chronic intravenous catheters are technically difficult to maintain for longitudinal studies. Hence, in this study, intraperitoneal administration of [¹⁸F]-FDOPA was evaluated as a less-invasive alternative that facilitates longitudinal imaging. Our experiments comprised the following assessments: (i) comparison of [¹⁸F]-FDOPA uptake between intravenous and intraperitoneal radiotracer administration and optimisation of the time window used for extended Patlak analysis, (ii) comparison of Ki ^Mod in a within-subject design of both administration routes, (iii) test-retest evaluation of Ki ^Mod in a within-subject design of intraperitoneal radiotracer administration, and (iv) validation of Ki ^Mod estimates by comparing the two administration routes in a mouse model of hyperdopaminergia induced by subchronic ketamine. Our results demonstrate that intraperitoneal [¹⁸F]-FDOPA administration resulted in good brain uptake, with no significant effect of administration route on Ki ^Mod estimates (intraperitoneal: 0.024 ± 0.0047 min^-1, intravenous: 0.022 ± 0.0041 min^-1, p = 0.42) and similar coefficient of variation (intraperitoneal: 19.6%; intravenous: 18.4%). The technique had a moderate test-retest validity (intraclass correlation coefficient (ICC) = 0.52, N = 6) and thus supports longitudinal studies. Following subchronic ketamine administration, elevated K _i ^Mod as compared to control condition was measured with a large effect size for both methods (intraperitoneal: Cohen's d = 1.3; intravenous: Cohen's d = 0.9), providing further evidence that ketamine has lasting effects on the dopamine system, which could contribute to its therapeutic actions and/or abuse liability.

Three-step two-pot automated production of NCA [18F]FDOPA with FlexLab module

Automated three-step two-pot production of no-carrier-added (NCA) [¹⁸F]FDOPA was first implemented in the iPHASE FlexLab module. Decay-corrected radiochemical yield (RCY) of [¹⁸F]FDOPA synthesized by this method was 10~14% (n = 7) with a synthesis time of ~110 min [¹⁸F]FDOPA was obtained in > 95% of radiochemical purity with a molar activity of ~431 GBq/μmol. With the method successfully implementing on the commercial FlexLab module and its built-in step-by-step activity monitoring, further processes optimization would be achieved.

Evaluation of exercise-induced modulation of glial activation and dopaminergic damage in a rat model of Parkinson's disease using [11C]PBR28 and [18F]FDOPA PET

Evidence suggests that exercise can modulate neuroinflammation and neuronal damage. We evaluated if such effects of exercise can be detected with positron emission tomography (PET) in a rat model of Parkinson's disease (PD). Rats were unilaterally injected in the striatum with 6-hydroxydopamine (PD rats) or saline (controls) and either remained sedentary (SED) or were forced to exercise three times per week for 40 min (EX). Motor and cognitive functions were evaluated by the open field, novel object recognition, and cylinder tests. At baseline, day 10 and 30, glial activation and dopamine synthesis were assessed by [¹¹C]PBR28 and [¹⁸F]FDOPA PET, respectively. PET data were confirmed by immunohistochemical analysis of microglial (Iba-1) / astrocyte (GFAP) activation and tyrosine hydroxylase (TH). [¹¹C]PBR28 PET showed increased glial activation in striatum and hippocampus of PD rats at day 10, which had resolved at day 30. Exercise completely suppressed glial activation. Imaging results correlated well with post-mortem Iba-1 staining, but not with GFAP staining. [¹⁸F]FDOPA PET, TH staining and behavioral tests indicate that 6-OHDA caused damage to dopaminergic neurons, which was partially prevented by exercise. These results show that exercise can modulate toxin-induced glial activation and neuronal damage, which can be monitored noninvasively by PET.

Lentiviral-mediated knock-down of GD3 synthase protects against MPTP-induced motor deficits and neurodegeneration

Converging evidence demonstrates an important role for gangliosides in brain function and neurodegenerative diseases. Exogenous GM1 is broadly neuroprotective, including in rodent, feline, and primate models of Parkinson's disease, and has shown positive effects in clinical trials. We and others have shown that inhibition of the ganglioside biosynthetic enzyme GD3 synthase (GD3S) increases endogenous levels GM1 ganglioside. We recently reported that targeted deletion of St8sia1, the gene that codes for GD3S, prevents motor impairments and significantly attenuates neurodegeneration induced by 1-methy-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). The current study investigated the effects of GD3S inhibition on the neurotoxicity and parkinsonism induced by MPTP. Mice were injected intrastriatally with a lentiviral-vector-mediated shRNA construct targeting GD3S (shGD3S) or a scrambled-sequence control (scrRNA). An MPTP regimen of 18 mg/kg x 5 days reduced tyrosine-hydroxylase-positive neurons in the substantia nigra pars compacta of scrRNA-treated mice by nearly two-thirds. In mice treated with shGD3S the MPTP-induced lesion was approximately half that size. MPTP induced bradykinesia and deficits in fine motor skills in mice treated with scrRNA. These deficits were absent in shGD3S-treated mice. These results suggest that inhibition of GD3S protects against the nigrostriatal damage, bradykinesia, and fine-motor-skill deficits associated with MPTP administration.

Dissociable effects of dopamine on learning and performance within sensorimotor striatum

Striatal dopamine is an important modulator of current behavior, as seen in the rapid and dramatic effects of dopamine replacement therapy in Parkinson Disease (PD). Yet there is also extensive evidence that dopamine acts as a learning signal, modulating synaptic plasticity within striatum to affect future behavior. Disentangling these "performance" and "learning" functions is important for designing effective, long-term PD treatments. We conducted a series of unilateral drug manipulations and dopamine terminal lesions in the dorsolateral striatum of rats highly-trained to perform brief instructed head/neck movements (two-alternative forced choice task). Reaction times and accuracy were measured longitudinally to determine if task behavior changed immediately, progressed over time, and/or persisted after drug withdrawal. Enhanced dopamine signaling with amphetamine caused an immediate, nonprogressive, and bilateral decrease in reaction times (RT). The altered RT distributions were consistent with reduced distance to threshold in the linear approach to threshold with ergodic rate (LATER) model of decision-making. Conversely, the dopamine antagonist flupenthixol caused experience-dependent, persistent changes in RT and accuracy indicative of a "learning" effect. These RT distributions were consistent with a slowed rate of approach to decision threshold. Our results show that dopaminergic signaling makes dissociable contributions to current and future behavior even within a single striatal subregion, and provide important clues for both models of normal decision-making and the design of novel drug therapies in PD.

Biomarkers in Parkinson's disease: an update

PURPOSE OF REVIEW

This review article is focused upon the most recent biomarker studies of Parkinson's disease. It provides an update on promising areas of biomarker research in a rapidly expanding field, and discusses future directions that might lead to successful development of Parkinson's disease biomarkers.

RECENT FINDINGS

Studies of molecular-genetic and biochemical biomarkers of Parkinson's disease have not only targeted hypothesis-driven measures of specific substrates involved in processes such as protein misprocessing, but also have made use of sophisticated analyses such as transcriptomic, proteomic, and metabolomic approaches. Whereas none of these are yet established as Parkinson's disease biomarkers, brain imaging using the 123I-ioflupane ligand with single-photon emission computed tomography was recently approved in the United States to aid in Parkinson's disease diagnosis, and research on other imaging modalities is ongoing. Neurophysiological tests are also being adapted for biomarker research, and we review recent promising data.

SUMMARY

The search for effective biomarkers for diagnosis and surveillance of Parkinson's disease continues. A battery of biomarkers comprising different modalities might be required to address clinical needs in this complex disorder. Critically, collaborative efforts including centralized tissue repository and clinical research infrastructure that are being organized will advance this field further.

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.