Mapping alteration of dopaminergic neurons in a rat model of Parkinson Disease through the comparison of the presynaptic PET tracers, [18F]-LBT999 and 6-[18 F]fluoro-L-m-tyrosine

ABSTRACT – EMIM 2018

Introduction

The pathological features of Parkinson’s Disease (PD) are driven by the loss of the dopaminergic projection neurons in the substantia nigra (SN) resulting in a dopamine (DA) deficiency in the striatum [1]. We developed a pathologically relevant rodent PD model; overexpressing the mutant (A53T) human alpha-synuclein protein in the SN [2]. Our aims are triple: 1- map neuronal loss and DA deficiency over time using two different presynaptic PET tracers, 2- evaluate the sensitivity of each radioligand, and 3- correlate individual PET data to behaviour and histological results.

Methods

A total of twelve rats were unilaterally injected in the SN with a viral vector (AAV2/6) overexpressing mutated (A53T) human alpha-synuclein, and were studied either at 6 weeks post-injection (6wpi, n=7, 543±36g) or 12wpi (n=4, 573±40g). PET imaging was performed using a ligand substrate for AADC, 6-[18F]fluoro-L-m-tyrosine (“FMT”, 60min acquisition, 31.3-60.8MBq; pre-treatment by IP injection of 10mg/kg benserazide 30’ before imaging [3]), and a ligand for DA transporter (DAT), [18F]-LBT999 [4](“LBT", 90min acquisition, 40.3-63.0MBq). For behaviour, rats were subjected for 5 minutes to the cylinder test, in which contralateral and ipsilateral paw use was compared. After the in vivo studies rats were sacrificed for histological studies using tyrosine hydroxylase immunohistochemistry.

Results/Discussion

From LBT and FMT PET scans, quantitative uptake images (BPnd and Ki) were calculated using Logan and Patlak graphical methods with the cerebellum as a reference. Unilateral AAV injections allowed the contralateral striatum to serve as internal control. Injection of benserazide was not effective in 36% of the animals, in which Ki could not reasonably be estimated. Additionally, Ki images showed more non-specific binding than BPnd images. At 12wpi we observed a decreased BPnd in the ipsilateral striatum, and similarly for the Ki of the quantifiable scans. These results are in concordance with the behavioural observations, showing roughly only 30% use of the contralateral forepaw. Absolute correlative analysis and histological comparisons are still ongoing. At 6wpi we did not observe any asymmetry with FMT, however preliminary LBT data suggest an asymmetric uptake in some animals. Absolute quantification is still ongoing; these results will be correlated with behaviour and histology.

Conclusions

We created an AAV rat model of PD that shows progressive DA deficiency and neuronal loss detectable by FMT and LBT PET imaging. Preliminary data suggest that the DAT tracer is more sensitive to detect a mild PD phenotype as compared to the AADC tracer. This phenomenon has previously been described, and is possibly due to a combination of reduced nerve terminal DAT binding sites and downregulation of DAT in surviving neurons, in an attempt to increase DA availability [5]. Further analysis of PET data will allow correlating PET data to behavioural and histological measurements.

 Acknowledgement

This project has been funded by the European Union Horizon 2020 Programme (H2020-MSCA-ITN-2015) under the Marie Skłodowska-Curie Innovative Training Network and Grant Agreement No. 676408.

References

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2.            Cresto, N., ; Gaillard M.C.; Joséphine, C.; Aurégan, G.; Guillermier, M.; Bernier, S.; Jan, C.; Petit, F. Gipchtein, P.; Joliot, A.; Hantraye, P.; Cambon, K.; Bemelmans, A.; Brouillet, E., THE LRRK2 G2019S MUTATION BUT NOT ITS DEAD KINASE FORM INCREASES THE NEUROTOXICITY OF MUTANT A53T A-SYNUCLEIN. Neurodegener Dis 2017;17(suppl 1):8-590 – Page 448, 2017.

3.            Becker, G., et al., Comparative assessment of 6-[18 F]fluoro-L-m-tyrosine and 6-[18 F]fluoro-L-dopa to evaluate dopaminergic presynaptic integrity in a Parkinson's disease rat model. J Neurochem, 2017.

4.            Serriere, S., et al., In vivo PET quantification of the dopamine transporter in rat brain with [(1)(8)F]LBT-999. Nucl Med Biol, 2014. 41(1): p. 106-13.

5.            Arena, J.E. and A.J. Stoessl, Optimizing diagnosis in Parkinson's disease: Radionuclide imaging. Parkinsonism Relat Disord, 2016. 22 Suppl 1: p. S47-51.