Of the acetylcholine muscarinic receptors, the type 1 (M1) and type 2 (M2) receptors are expressed at the highest levels in the prefrontal cortex (PFC) and hippocampus, brain regions important for cognition. As equivocal findings of age-related changes of M1 and M2 in the nonhuman primate brain have been reported, we first assessed age-related changes in M1 and M2 in the PFC and hippocampus using saturation binding assays. Maximum M1 receptor binding, but not affinity of M1 receptor binding, decreased with age. In contrast, the affinity of M2 receptor binding, but not maximum M2 receptor binding, increased with age. To determine if in the elderly cognitive performance is associated with M1 or M2 function, we assessed muscarinic function in elderly female rhesus macaques in vivo using a scopolamine challenge pharmacological magnetic resonance imaging and in vitro using saturation binding assays. Based on their performance in a spatial maze, the animals were classified as good spatial performers (GSP) or poor spatial performers (PSP). In the hippocampus, but not PFC, the GSP group showed a greater change in T(2)*-weighted signal intensity after scopolamine challenge than the PSP group. The maximum M1 receptor binding and receptor binding affinity was greater in the GSP than the PSP group, but no group difference was found in M2 receptor binding. Parameters of circadian activity positively correlated with the difference in T(2)*-weighted signal intensity before and after the challenge, the maximum M1 receptor binding, and the M1 receptor binding affinity. Thus, while in rhesus macaques, there are age-related decreases in M1 and M2 receptor binding, in aged females, hippocampal M1, but not M2, receptor function is associated with spatial learning and memory and circadian activity.

Funding Acknowledgements Type of funding sources: Public Institution(s). Main funding source(s): National Institute of Health Objectives Patients with Alzheimer’s Disease (AD) display cerebral white matter (WM) hyperintensities in association with microvascular brain injury (mVBI) and cerebral microinfarcts. We determined the extent to which vasomotor dysfunction of WM penetrating arterioles is associated with mVBI in autopsy brains from patients with low or high AD neuropathology, and determined if these changes correlate with quantitative indices of MRI-defined WM integrity. Methods We analyzed 28 consecutive cases of prefrontal rapid autopsies in a population-based cohort with low or high AD neuropathological changes, and with or without cerebral microinfarcts (mVBI). WM penetrating and pial surface arteriolar responses to the endothelium-dependent agonist, bradykinin were assessed ex vivo with videomicroscopy. Expression of vascular endothelial nitric oxide synthase (eNOS) and NAD(P)H-oxidase (Nox1,2 and 4 isoforms) was measured with quantitative PCR. Diffusion tensor imaging was used to measure mean apparent diffusion coefficient (ADC) and fractional anisotropy (FA) in post-mortem prefrontal WM. Results Patients with high AD neuropathology and mVBI exhibited a significantly reduced dilation in response to bradykinin selectively in WM arterioles, when compared to low or high AD cases without mVBI or to pial surface arterioles. Expression of eNOS was reduced, whereas Nox-1 expression was increased in WM arterioles in AD and mVBI cases. Moreover, we found that in cases with low AD pathology the magnitude of bradykinin-induced WM arteriole dilation was correlated with higher FA and lower ADC. In contrast, dilation to bradykinin was associated with lower FA and higher ADC in cases with high AD neuropathology. Interpretation: Selectively impaired vasodilation to the endothelium-dependent agonist, bradykinin occurs in WM penetrating arterioles in AD patients with WM microinfarcts, which is likely due to reduced nitric oxide and an increase in Nox1-derived reactive oxygen species production. Notably, the bradykinin response of WM arterioles strongly correlated with MRI-defined WM integrity suggesting that impaired bradykinin-mediated vasodilation in WM penetrating arterioles contributes to disrupted white matter microstructural integrity as defined by MRI.