Therapeutic effects of acoustic stimulation at gamma band frequency in a mouse model of Alzheimer’s disease

Abstract

The accumulation of amyloid-beta (Aβ) is recognized as one of the neuropathologic hallmarks of Alzheimer’s disease (AD) and abnormal neurophysiologic findings have been observed such as abnormal gamma band oscillations and brain connectivity. Recently, the effects of gamma frequency entrainment on decreasing Aβ were reported using optogenetic and visual stimulations. However, the affected areas were limited to the hippocampus and visual cortex, although the therapeutic target areas should be a wider area of the brain, including the frontal cortex. Therefore, we investigated the effects of gamma entrainment using acoustic stimulation and optogenetic stimulation of the basal forebrain parvalbumin neurons that are known to regulate the cortical gamma oscillations. We used 5XFAD and 5XFAD/PV-Cre mice recapitulating major features of AD for acoustic and optogenetic stimulation, respectively. Electroencephalogram (EEG) electrodes were chronically placed on frontal and parietal sites of the skull of freely moving mice. Acoustic and optogenetic stimulation at 40 Hz were given for 2 hours per day for 2 weeks at the age of 6 months. The number of Aβ plaque and microglia was evaluated by immunohistochemistry analysis and the amount of soluble and insoluble Aβ peptides was assessed by ELISA. Evoked and spontaneous gamma power were analyzed by wavelet analysis. Coherence and phase locking value (PLV) analysis were used to assess fronto-parietal connectivity. The number of Aβ plaques decreased in the pre- and infra- limbic (PIL) and hippocampus regions (46.8% and 60.0% respectively) and soluble Aβ-40 and -42 peptides (45.5% and 67.2%, respectively) in PIL in the acoustic stimulation group compared to the untreated group. We also found that the number of activated microglia increased in PIL and hippocampus (188.3% and 98.9%, respectively). In EEG analysis, 5XFAD mice showed an increased evoked gamma power at 40 Hz, but it returned to the similar level to those of the control mice after acoustic stimulation. Conversely, spontaneous gamma power increased during the stimulation period. Connectivity analysis showed that the evoked gamma coherence did not differ between 5XFAD mice with and without acoustic stimulation, whereas PLV in spontaneous gamma oscillations showed a increasing pattern through the stimulation period in the acoustic stimulation group. Optogenetic stimulation of the basal forebrain parvalbumin neurons also resulted in a decreased level of evoked gamma power. In summary, we found that acoustic stimulation at 40 Hz for 2 weeks in 5XFAD can reduce Aβ plaque and peptides in the brain and restore abnormal neurophysiological pattern, including the gamma band oscillations and the fronto-parietal connectivity. Our data suggest that acoustic stimulation at 40 Hz might alter the natural deterioration processes of AD and have a therapeutic potential in AD.