Hippocampal beta-amyloid reduces locus coeruleus glutamate and tyrosine hydroxylase

The effects of intrahippocampally injected beta-amyloid protein (beta-AP) on glutamate- (Glu) and tyrosine hydroxylase (TH)-like immunoreactivities in the neurons of the locus coeruleus (LC) were studied in rats. A synthetic peptide or the vehicle alone was injected into the hippocampus as controls. All injections were made once a week (two or three injections; 3 nmol in 2 microliters of distilled water). Fluorescent microspheres (either alone or with one of the peptides) were also injected into the hippocampus to identify coeruleo-hippocampal neurons. The results revealed cell loss in the hippocampus at the site near beta-AP or control peptide deposition. Furthermore, in beta-AP/microsphere injected animals, only 22.4% and 49.6% of hippocampal projection neurons contained Glu and TH, respectively, compared to 88.4% and 85.3% in the animals that received control peptide with microspheres. Our results suggest that beta-AP has an effect on noradrenergic cells whose axons project to the hippocampus. These effects may contribute to the TH cell loss in the LC of Alzheimer's brains.