Monitoring synaptic loss in a rat model of stroke: a PET imaging study with [¹⁸F]SynVesT-2, radiotracer for the synaptic vesicle protein 2A (SV2A)

Introduction: Stroke is a devastating disease with high mortality and morbidity rates. FDG PET has been used to probe brain tissue viability (penumbra) after acute ischemic stroke;¹ while MR and CT have been used to image infarct through anatomical and perfusion changes.² However, the transient hyperglycemia, a common phenomenon in acute stroke patients, could affect the FDG uptake and thereby confounding its interpretation.¹ Because synapse is a crucial microstructure for brain functions and synaptic deficit is a hallmark of stroke, we, for the first time, tested in vivo PET imaging using our recently developed SV2A tracer [¹⁸F]SynVesT-2 to track the progression of synapse loss in a rat model of stroke.

Methods: The stroke model was established through the middle cerebral artery occlusion (MCAO) procedure, with 4 h occlusion followed by reperfusion.³ A total of 6 stroke model rats underwent weekly PET scans for four weeks starting at 1-day post reperfusion. The SV2A PET ligand [¹⁸F]SynVesT-2 was synthesized via iodonium ylide precursor and was administered intravenously to the rats. Emission data were acquired from 0-60 min post-injection on the FOCUS 220 scanners. Images were reconstructed with attenuation and scatter correction. The Waxholm Space (WHS) rat brain template was manually registered to PET images to generate standardized uptake values (SUVs) for hippocampus, cerebellum, neocortex, and thalamus. The SUVs of ipsilateral brain regions averaged from either 30-60 min or 0-5 min were normalized with the SUVs of the contralateral regions to generate SUV ratio (SUVR). We chose the SUVR from later time window (30-60 min post tracer injection) for SV2A quantification, and early time window (0-5 min) as a surrogate of perfusion. The volume of lesion was assessed using SUVR normalized by cerebellum, after smoothing, masking, subtracting, thresholding, and binarizing. One-way repeated measure ANOVA and t-test were used in the statistical analysis.

Results: Using the contralateral brain regions as reference regions, the SUVR_{(30-60 min)} of the infarct core regions (hippocampus, thalamus and neocortex) decreased over time (p < 0.01), most significantly from 1-day to 7-day post reperfusion (p < 0.006); but remained unchanged in cerebellum (p > 0.1). As a readout for blood flow, the SUVR_{(0-5 min)}in the ischemic core regions was robustly reduced by ~50% at later time points; but highly variable at one-day post reperfusion, which might reflect a transient bilateral blood brain barrier opening after stroke (Figure 1A,B). Both the parametric SUVR_{(30-60 min)} PET images (Figure 1C) and the infarct maps clearly identified the brain lesion regions and sizes. Based on the infarct lesion maps (Figure 1D), the infarct volume consistently increased over time (p < 0.01), most significantly from 1-day to 7-day post stroke (p = 0.0003; 0.037 ± 0.049 mL, 0.24 ± 0.11 mL, 0.34 ± 0.16 mL, and 0.38 ± 0.09 mL, at 1-day, 1-week, 2-week, and 3-week post reperfusion, respectively).

Conclusions: Using [¹⁸F]SynVesT-2 and PET imaging, we successfully detected synapse loss in rat MCAO model and tracked disease progression via lesion quantification. Synapse loss is mainly in hippocampus, thalamus, and neocortex, rather than cerebellum. Furthermore, most significant loss of synapses was observed to occur during the first week post reperfusion. Our results showed that the quantification of synaptic density by SV2A PET may provide an objective measure of disease progression and a primary endpoint to evaluate treatment efficacy of novel therapeutics for stroke in clinical trials. References:

1. Bunevicius et al. Biomed Res Int 2013. 2013: 634598.

2. Saver. Stroke 2011, 42: 2356-2364.

3. Cai et al. Neurosci Lett 2015, 597: 127-31.

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