Evaluation of CB2R expression and pyridine-based radiotracers in brains from a mouse model of Alzheimer’s disease

Neuroinflammation plays an important role in the pathophysiology of Alzheimer’s disease. The cannabinoid type 2 receptor (CB 2 R) is an emerging target for neuroinflammation and therapeutics of Alzheimer’s disease. Here, we aimed to assess the alterations in brain CB 2 R levels and evaluate novel CB 2 R imaging tracers in the arcAβ mouse model of Alzheimer’s disease amyloidosis. Immunohistochemical staining for Aβ deposits (6E10), microgliosis (anti-Iba1 and anti-CD68 antibodies), astrocytes (GFAP) and the anti-CB 2 R antibody was performed on brain slices from arcAβ mice 17 months of age. Autoradiography using the CB 2 R imaging probes [ 18 F]RoSMA-18-d6, [ 11 C]RSR-056 and [ 11 C]RS-028 and mRNA analysis were performed in brain tissue from arcAβ and nontransgenic littermate (NTL) mice at 6, 17, and 24 months of age. Specific increased CB 2 R immunofluorescence intensities on the increased number of GFAP-positive astrocytes and Iba1-positive microglia were detected in the hippocampus and cortex of 17-month-old arcAβ mice compared to NTL mice. CB 2 R immunofluorescence was higher in the glial cells inside 6E10-positive amyloid-β deposits than peri-plaque with a low background. Ex vivo autoradiography showed that the binding of [ 18 F]RoSMA-18-d6 and [ 11 C]RSR-056 was comparable in arcAβ and NTL mice at 6, 17 and 24 months. The level of Cnr2 mRNA expression in the brain was not significantly different between arcAβ and NTL mice at 6, 17 or 24 months. In conclusion, we demonstrated pronounced specific increases in microglial and astroglial CB 2 R expression levels in a model of AD-related cerebral amyloidosis/AD mouse model, emphasizing CB 2 R as a suitable target for imaging neuroinflammation.