[18F]Nifene binding to Nicotinic α4β2* Receptors in Anterior Cingulate of Postmortem Human Alzheimer’s Disease Brain

1537 Objectives: Anterior cingulate (AC) in the human brain is innervated with nicotinic α4β2* receptors in the human brain. This receptor system is involved in cognition, learning and memory and may be affected in neurodegeneration. In our efforts to translate use of human [18F]Nifene PET in Alzheimer’s disease (AD), we have carried out quantitative autoradiographic evaluation of nicotinic α4β2* receptors using AC in postmortem AD brains using [18F]Nifene. Methods: Human post-mortem brain tissues consisting of AC and corpus callosum, CC (AD, n=6, age 77-89, SP Stage C and controls (CN), n=6; age 81-90 SP Stage 0-A) were obtained from Banner Health, Sun City, Arizona. Brain slices (10 μm thick) consisting of both AC and CC of each subject were obtained on a Leica 1850 cryotome. Adjacent brain slices were incubated [18F]Nifene (1 μCi/cc) in Tris/pH 7.4 buffer at 25 oC for 1 hr. Nonspecific binding was measured using 300 μM nicotine. Adjacent sections were tested for neurofibrillary tangles (NFT) using [18F]FAZIN3 (1 μCi/cc) in PBS buffer at 25 oC for 1 hr. Using the Optiquant program (Packard Instruments Co), regions of interest were drawn and digital light units/ mm2 (DLU/mm2) were used to quantify the percentage change in binding of the radiotracers. Results: All CN subjects exhibited significant [18F]Nifene binding in the AC whereas CC had very low levels of [18F]Nifene. This is consistent with our PET [18F]Nifene results in healthy human subjects with distribution volume ratios (DVR) greater than 1.30 (using CC as reference) were found for AC. [18F]FAZIN3 showed significant binding in AD brain slices confirming presence of NFT, while CN brains had lower binding of [18F]FAZIN3. For [18F]Nifene, the ratio for AC between AD and CN was 0.31, suggesting a significant reduction in [18F]Nifene binding in AD. Binding of [18F]Nifene in the CC was also found to be lower in AD compared to CN brain samples. Nicotine displaced [18F]Nifene binding from these brain regions. These findings are consistent with our previous observation suggesting a decreased binding of [3H]Cytisine in AC in the brain samples of AD. Conclusions: Our results indicate that nicotinic α4β2* receptors are reduced significantly in anterior cingulate of the AD brain. The extent of reduction is significantly greater in AD than the 5-10% test-retest error found in human [18F]Nifene PET. This suggests that [18F]Nifene may be a useful PET probe for AD. Further correlative studies of [18F]Nifene binding in gray and white matter regions of AD are underway using fluorescent probe, nifrorhodhamine in order to understand cellular nature of changes in nicotinic α4β2* receptors in AD. Research Support: NIH/NIA RF1 AG029479 (JM)