Microstructural integrity of the locus coeruleus and its tracts reflect noradrenergic degeneration in Alzheimer’s and Parkinson’s disease

Abstract

Abstract Background Degeneration of the locus coeruleus (LC) noradrenergic system contributes to clinical symptoms in Alzheimer’s (AD) and Parkinson’s disease (PD). Diffusion MRI has potential to evaluate the integrity of the LC noradrenergic system. The aim of the current study was to determine whether diffusion MRI-measured integrity of the LC and its tract is sensitive to noradrenergic degeneration in AD and PD.Methods Post-mortem in-situ T1-weighted and multi-shell diffusion MRI were performed for 9 AD, 14 PD, and 8 control brain donors. Fractional anisotropy (FA) and Mean diffusivity (MD), were derived from the LC, and from tracts between the LC and anterior cingulate cortex, dorsolateral prefrontal cortex (DLPFC), primary motor cortex (M1) and hippocampus. Brain tissue sections of the LC and cortical regions were obtained and immunostained for dopamine-beta hydroxylase (DBH) to quantify noradrenergic cell density and fiber load. Group comparisons and correlations between outcome measures were performed using linear regression and partial correlations.Results AD and PD cases showed loss of LC noradrenergic cells and fibers. In the cortex, AD cases showed increased DBH + immunoreactivity in the DLPFC compared to PD cases and controls, while PD cases showed reduced DBH + immunoreactivity in the M1 compared to controls. Higher FA within the LC was found for AD, which was correlated with loss of noradrenergic cells and fibers in the LC. Increased FA of the LC-DLPFC tract was correlated with LC noradrenergic fiber loss in the combined AD and control group, whereas the LC-M1 tract was correlated with LC noradrenergic neuronal loss in the combined PD and control group. Tract alterations were not correlated with cortical DBH + immunoreactivity.Conclusions In AD and PD, diffusion MRI may have the potential to capture noradrenergic-related alteration within the LC and its tract, driven by local noradrenergic neuronal loss within the LC, rather than noradrenergic changes in the cortex.