Background: Ischemic stroke is a devastating disease characterized by the sudden loss of cerebral blood flow, leading to hypoxic-ischemic injury of brain, and resulting in neurological deficit. Due to the weak regenerative capacity of brain tissue and limited therapeutic arsenal, stroke remains the leading cause of death and disability worldwide [1]. Synaptic loss is a crucial hallmark of stroke, profoundly affecting the deficit of neurological functions. Establishing a noninvasive and quantitative methodology for evaluating synapse in vivo would facilitate the development of future physical and pharmacological strategies to enhance recovery after stroke [2]. [¹⁸F]fluorodeoxyglucose (FDG) is the most widely used radiopharmaceutical to assess cerebral glucose metabolism. As a significant proportion of glucose consumption is closely coupled with synapse signaling, decreased [¹⁸F]FDG uptake can indicate a deficit in synaptic activity and neural function [3, 4]. However, it has been recognized that factors such as microglia activation and astroglia glutamate transport may influence [¹⁸F]FDG uptake in various neurological disorders, so that the significance of increased [¹⁸F]FDG uptake remains controversial [5]. Recently, radiotracers targeting synaptic vesicle glycoprotein 2 subtype A (SV2A), such as [¹¹C]UCB-J and [¹⁸F]SynVesT-1, have been developed for measuring synaptic density in vivo, and shown favorable ability to quantifying dynamic synaptic changes during disease progression [6, 7]. However, the potential of synaptic imaging in assessing stroke has been rarely reported.

Objective: [¹⁸F]SynVesT-1 is a novel radiopharmaceutical for assessing synaptic density in vivo. This study aims to investigate the potential of [¹⁸F]SynVesT-1 PET in evaluating neurological recovery in the rat model of ischemic stroke, and to compare its performance with [¹⁸F]FDG PET.

Methods: Sprague-Dawley rats were subjected to photothrombotic cerebral infarction, and safinamide was injected intraperitoneally from day 3 to day 14 post-stroke to alleviate neurological deficit. Cylinder test and forelimb placing test were performed to assess the neurological function. MRI, [¹⁸F]SynVesT-1 PET/CT and [¹⁸F]FDG PET/CT imaging were used to evaluate infarct volume, synaptic density and cerebral glucose metabolism pre- and post-treatment, respectively. [¹⁸F]SynVesT-1 and [¹⁸F]FDG PET images were compared with Statistical Parametric Mapping (SPM)- and region of interest (ROI)-based analysis. Post-mortem histological analysis including immunostaining and Golgi‐Cox staining were performed to validate PET images.

Results: Safinamide treatment improved behavioral outcomes in stroke-damaged rats. Both [¹⁸F]SynVesT-1 and [¹⁸F]FDG PET could detect the stroke-caused injury, while the injured region was significantly larger in [¹⁸F]FDG PET than [¹⁸F]SynVesT-1 PET (16047 voxels vs. 1994 voxels). Compared with the saline group, radiotracer uptake in the injured area was significantly increased in [¹⁸F]SynVesT-1 PET after safinamide treatment, whereas no notable change was observed in [¹⁸F]FDG PET. In addition, [¹⁸F]SynVesT-1 PET imaging presented a better correlation with the recovery of neurological functions than [¹⁸F]FDG PET. Consistent with PET results, the intensity of SV2A in the safinamide group was significantly higher than that in the saline group (P < 0.01) and GLUT1-immunostaining showed no differences between two groups (P = 0.062). Golgi‐Cox staining results indicated that safinamide treatment enhanced dendritic complexity and structural neuroplasticity after stroke. In addition, immunostaining indicated that safinamide treatment decreased the expression of GFAP (P < 0.01) and the number of Iba+ microglia (P < 0.05), while also increasing the number of NeuN-labeled neurons in the ischemia-injured area (P < 0.01).

Conclusions: [¹⁸F]SynVesT-1 PET was able to quantify spatiotemporal dynamics of synaptic density in the rat model of stroke, and showed different ability to detect stroke injury and neurological recovery compared with [¹⁸F]FDG PET. The utilization of [¹⁸F]SynVesT-1 PET holds promise as a potential non-invasive biomarker for evaluating ischemic stroke in conjunction with [¹⁸F]FDG PET.

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Image/Figure Caption:

(a) Voxel-based analysis of decreased [¹⁸F]FDG and [¹⁸F]SynVesT-1 uptake of pre-treatment stroke-injured rats (n = 19) compared to the normal rats (n = 6). Figure shows clusters with extent threshold > 200 voxels, P < 0.001 (uncorrected). The T-maps are overlaid on template T2WI MRI enclosed in Small Animal Molecular Imaging Toolbox.

(b) Representative T2WI MRI, [¹⁸F]FDG, [¹⁸F]SynVesT-1 PET image of pre-treatment stroke-injured rats. Blue circles showed the infraction ROIs based on T2WI MRI of 2 days after stroke.

(c) Quantification of the ratio voxels over a certain threshold (50% SUV_max) in the ROIs in [¹⁸F]FDG and [¹⁸F]SynVesT-1 PET imagings of pre-treatment stroke-injured rats (n = 19). SUV_max = maximum SUV of contralateral cortex.

(d) Representative immunostaining of GLUT1 and SV2A of pre-treatment stroke-injured rats. Scale bars=50 μm.

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