Age-dependent changes of calcium related activity in the central auditory pathway

• The study determines calcium activity in the brains of aging mice over the lifespan.
• Activity increase is particularly present in middle-aged groups (13 to 18 months).
• Activity drop down to control levels in oldest animals (22 months).
• Changes might be related to neuroplasticity, hyperactivity and neurodegeneration.
• Results represent central nervous correlates of age-related hearing disorders.

Age-related hearing loss (ARHL) represents one of the most common chronic health problems that faces an aging population. In the peripheral auditory system, aging is accompanied by functional loss or degeneration of sensory as well as non-sensory tissue. It has been recently described that besides the degeneration of cochlear structures, the central auditory system is also involved in ARHL. Although mechanisms of central presbycusis are not well understood, previous animal studies have reported some signs of central neurodegeneration in the lower auditory pathway. Moreover, changes in neurophysiology are indicated by alterations in synaptic transmission. In particular, neurotransmission and spontaneous neuronal activity appear to be affected in aging animals. Therefore, it was the aim of the present study to determine the neuronal activity within the central auditory pathway in aging mice over their whole lifespan compared to a control group (young adult animals, ~ 3 months of age) using the non-invasive manganese-enhanced MRI technique. MRI signal strength showed a comparable pattern in most investigated auditory brain areas.An increase in activity was particularly pronounced in the middle-aged groups (13 or 18 months), with the largest effect in the dorsal and ventral cochlear nucleus. In higher auditory structures, namely the inferior colliculus, medial geniculate body and auditory cortex, the enhancement was much less expressed; while a decrease was detected in the superior olivary complex. Interestingly, calcium-dependent activity reduced to control levels in the oldest animals (22 months) in the cochlear nucleus and was significantly reduced in higher auditory structures. A similar finding was also found in the hippocampus. The observed changes might be related to central neuroplasticity (including hyperactivity) as well as neurodegenerative mechanisms and represent central nervous correlates of the age-related decline in auditory processing and perception.