Comparison of AT8 immunoreactivity in the locus ceruleus and hippocampus of 154 brains from routine autopsies.

We compared semiquantitatively AT8 immunoreactivity in the locus ceruleus (LC) and hippocampus of 154 brains from routine autopsies to investigate the initial sites of phosphorylated tau (phospho-tau) development. The numbers of AT8-positive neurons and the severity of AT8-positive neuropil threads (NTs) in the LC were strongly associated: there were no cases with AT8-positive neurons that lacked NTs and 20 cases (13%) had only NTs in the LC. Phospho-tau pathologies in the LC were almost equally on both sides, although some cases (7.8%) showed unilateral predominance. The numbers of AT8-positive neurons in the LC and the numbers of AT8-positive neurons and NTs in the hippocampus were also strongly associated. There were only two cases with AT8-positive neurons in the LC that lacked phospho-tau pathology in the hippocampus, and 21 cases (13.6%) with phospho-tau pathology in the hippocampus that lacked AT8-positive neurons in the LC. The numbers of AT8-positive NTs in the LC and AT8-positive neurons and NTs in the hippocampus were also strongly associated. There were seven cases (4.5%) with AT8-positive NTs in the LC that lacked phospho-tau pathology in the hippocampus, and five cases (3.2 %) with phospho-tau pathologies in the hippocampus that lacked AT8-positive NTs in the LC. In this study, we could not confirm that phospho-tau pathologies begin in the LC. We suspect their simultaneous occurrences in both hippocampal regions and in LC.