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