Noise trauma alters D-[3H]aspartate release and AMPA binding in chinchilla cochlear nucleus

Exposure of adults to loud noise can overstimulate the auditory system, damage the cochlea, and destroy cochlear nerve axons and their synaptic endings in the brain. Cochlear nerve loss probably results from the death of cochlear inner hair cells (IHC). Additional degeneration in the cochlear nucleus (CN) is hypothesized to stem from overstimulation of the system, which may produce excitotoxicity. This study tested these predictions by exposing one ear of anesthetized adult chinchillas to a loud noise, which damaged the ipsilateral cochlea and induced degeneration in the glutamatergic cochlear nerve. During the first postexposure week, before cochlear nerve axons degenerated, glutamatergic synaptic release in the ipsilateral CN was elevated and uptake was depressed, consistent with hyperactivity of glutamatergic transmission and perhaps with the operation of an excitotoxic mechanism. By 14 days, when cochlear nerve fibers degenerated, glutamatergic synaptic release and uptake in the CN became deficient. By 90 days, a resurgence of transmitter release and an elevation of AMPA receptor binding suggested transmission upregulation through plasticity that resembled changes after mechanical cochlear damage. These changes may contribute to tinnitus and other pathologic symptoms that precede and accompany hearing loss. In contrast, the other ear, protected with a silicone plug during the noise exposure, exhibited virtually no damage in the cochlea or the cochlear nerve. Altered glutamatergic release and AMPA receptor binding activity in the CN suggested upregulatory plasticity driven by signals emanating from the CN on the noise-exposed side.