UC Irvine

Since cholinergic dysfunction has been implicated in Alzheimer's disease (AD), the effects of Aβ plaques on nicotinic acetylcholine receptors (nAChRs) α4β2* subtype were studied using the transgenic 5xFAD mouse model of AD. Using the PET radiotracer [¹⁸ F]nifene for α4β2* nAChRs, in vitro autoradiography and in vivo PET/CT studies in 5xFAD mice were carried out and compared with wild-type (C57BL/6) mice. Ratios of [¹⁸ F]nifene binding in brain regions versus cerebellum (CB) in 5xFAD mice brains were for thalamus (TH) = 17, hippocampus-subiculum = 7, frontal cortex (FC) = 5.5, and striatum = 4.7. [¹²⁵ I]IBETA and immunohistochemistry (IHC) in 5xFAD brain slices confirmed Aβ plaques. Nicotine and acetylcholine displaced [¹⁸ F]nifene in 5xFAD mice (IC₅₀ nicotine = 31-73 nM; ACh = 38-83 nM) and C57BL/6 (IC₅₀ nicotine = 16-18 nM; ACh = 34-55 nM). Average [¹⁸ F]nifene SUVR (CB as reference) in 5xFAD mice was significantly higher in FC = 3.04 compared to C57BL/6 mice FC = 1.92 (p = .001), whereas TH difference between 5xFAD mice (SUVR = 2.58) and C57BL/6 mice (SUVR = 2.38) was not significant. Nicotine-induced dissociation half life (t_1/2 ) of [¹⁸ F]nifene for TH were 37 min for 5xFAD mice and 26 min for C57BL/6 mice. Dissociation half life  for FC in C57BL/6 mice was 77 min , while no dissociation of [¹⁸ F]nifene occurred in the medial prefrontal cortex (mFC) of 5xFAD mice. Coregistration of [¹⁸ F]nifene PET with MR suggested that the mPFC, and anterior cingulate (AC) regions exhibited high uptake in 5xFAD mice compared to C57BL/6 mice. Ex vivo [¹⁸ F]nifene and in vitro [¹²⁵ I]IBETA Aβ plaque autoradiography after in vivo PET/CT scan of 5xFAD mouse brain were moderately correlated (r² = 0.68). In conclusion, 5xFAD mice showed increased non-displaceable [¹⁸ F]nifene binding in mPFC.