Hearing loss as a risk factor for cognitive impairment and loss of synapses in the hippocampus.

Although epidemiological studies have identified an association between hearing loss and cognitive impairment, there is a lack of biological evidence detailing the mechanisms underlying this association. The present study investigated the effects of hearing loss on cognitive impairment using an at-risk model. In this animal model, amyloid-ß (Aß) was administered to the brain to such an extent that it did not cause cognitive impairments but made the brain vulnerable to risk factors. This study included four experimental groups based on hearing level and Aß administration. Behavioral tests were conducted to evaluate cognitive function, and synaptic protein levels were measured in the hippocampus and prefrontal cortex. The group with hearing loss and Aß administration showed significantly greater deficits on cognitive tests associated with the hippocampus than the other three groups (only Aß administration, only hearing loss, and without hearing loss or Aß administration). The hearing loss and Aß administration group also had significantly lower levels of synaptic proteins in the hippocampus than the other groups. The present results suggest that hearing loss may act as a risk factor for cognitive impairment in Alzheimer's disease. Additionally, the present findings indicate hearing loss may cause hippocampal synapses to be more vulnerable to Aß-induced damage.