Effects of optogenetic locus coeurleus stimulation on functional connectivity in a rat model of Alzheimer’s disease

Abstract

AbstractBackgroundActivity of the locus coeruleus (LC) influences behaviors that go awry in prodromal Alzheimer’s disease (AD), and these abnormalities coincide with early accumulation of hyperphosphorylated tau in the LC. LC stimulation changes global functional connectivity, but it is unknown how AD‐like neuropathology affects the LC’s ability to regulate brain state. Therefore, we combined optogenetics and functional magnetic resonance imaging (fMRI) in the TgF344‐AD rats that develop endogenous hyperphosphorylated tau in the LC prior to amyloid or tau pathology in forebrain regions.MethodsAt 2 months of age, TgF344‐AD rats and wild‐type (WT) littermates were infused intra‐LC with a ChR2‐mCherry or an mCherry virus under the noradrenergic‐specific PRSx8 promoter (n=3/group). 2‐4 weeks prior to fMRI, rats were implanted with an optic fiber targeting the LC, and functional stimulation was confirmed using pupillometry. At approximately 6 months, rats underwent fMRI scans with and without 2Hz optogenetic stimulation. During scanning, rats were intubated and subcutaneously given 1.5mg/kg*hr pancuronium while anesthetized with 1.3% isoflurane. fMRI scans were gradient echo EPI (TR=1.25s) and obtained using a 9.4T Bruker Animal MRI scanner. Data were preprocessed using a custom Rodent Whole‐Brain fMRI Data Preprocessing Toolbox. Functional connectivity matrices were generated in MATLAB using Pearson’s linear correlations between parcellated regions of interest (ROI). Histograms were generated in MATLAB by collecting the fisher z‐transformed correlation coefficients across all ROIs for the averages of each group.ResultsPupillometry confirmed optogenetic activation of the LC in ChR2 animals but not mCherry controls. Similar functional connectivity was observed between Tg and WT animals. We next subtracted baseline fisher z‐transformed correlation coefficients from those during 2Hz stimulation within each genotype to determine whether LC stimulation shifted functional connectivity. Across all groups, effects of 2Hz LC stimulation were negligible.ConclusionsTg and WT rats had similar resting‐state functional connectivity at 6 months, and 2Hz LC optogenetic stimulation did not impact functional connectivity. However, functional connectivity deficits progress with age in this strain, and the 2Hz stimulation falls within normal LC pacemaker firing (∼0.5‐2Hz). Therefore, future work will incorporate different stimulation frequency, patterns, and older rats.