Decreased coherence between the two olfactory bulbs in Alzheimer's disease model mice.

Alzheimer's disease (AD) is a progressive, neurodegenerative disease characterized by cell and synapse losses and the ensued network disconnections. Olfactory impairment is common and occurs early in AD patients, suggesting its potential role in the early diagnosis of AD. However, the functional connectivity of the olfactory system in AD has received little attention. Using local field potentials symmetrically recorded from the two olfactory bulbs in awake mice, we found that in AD model mice with robust amyloid-β burden, the coherence between the two bulbs for spontaneous activities was significantly lower in all of the four frequency bands (theta, beta, low and high gamma: 2-12, 15-35, 35-65 and 65-100 Hz, respectively). Moreover, the coherence for odor-evoked activities was also lower, particularly in the beta band, when compared to their wild-type littermates. These results demonstrate that the inter-bulbar functional connectivity is impaired in AD mice, indicating a failure of brain activation in beta-related functions, which might be of importance in the diagnosis of AD.