Requirement of nicotinic acetylcholine receptor subunit β2 in the maintenance of spiral ganglion neurons during aging

Age-related hearing loss (presbycusis) is a major health concern for the elderly. Loss of spiral ganglion neurons (SGNs), the primary sensory relay of the auditory system, is associated consistently with presbycusis. The causative molecular events responsible for agerelated loss of SGNs are unknown. Recent reports directly link age-related neuronal loss in cerebral cortex with the loss of high-affinity nicotine acetylcholine receptors (nAChRs). In cochlea, cholinergic synapses are made by olivocochlear efferent fibers on the outer hair cells that express alpha 9 nAChR subunits and on the peripheral projections of SGNs that express alpha 2, alpha 4-7, and beta 2-3 nAChR subunits. A significantly decreased expression of the beta 2 nAChR subunit in SGNs was found specifically in mice susceptible to presbycusis. Furthermore, mice lacking the beta 2 nAChR subunit (beta 2-/-), but not mice lacking the alpha 5 nAChR subunit (alpha 5-/-), have dramatic hearing loss and significant reduction in the number of SGNs. Our findings clearly established a requirement for beta 2 nAChR subunit in the maintenance of SGNs during aging.