Temporary Versus Permanent Synaptic Loss from Repeated Noise Exposure in Guinea Pigs and C57 Mice.

A single brief noise exposure can cause a significant loss of cochlear afferent synapses without causing permanent threshold shift. Previously we reported that the initial synaptic loss is partially reversible in Guinea pigs, indicating that synaptic loss can be categorized as either temporary or permanent. Since synaptic loss is biased to innervating auditory nerve fibers (ANFs) with low spontaneous spike rates (SSR), which are critical to the coding of in-background noise, coding-in-noise deficits (CIND) have been predicted to result from noise-induced synaptic damage. However, recent study of the noise masking of amplitude-modulation (AM) evoked compound action potentials (CAP) tailed to find evidence for such deficits in either mice or Guinea pigs. The present study sought to determine the effects of repeated noise exposure on temporary and permanent synaptic loss in Guinea pigs and C57 mice, whether such effects were additive, and whether repeated noise exposure induced CIND in Guinea pigs. The results show that the second noise exposure caused much less temporary synaptic loss and no additional permanent loss in Guinea pigs; however, an additional permanent loss was seen after the second noise was in the mice, although it was not significant. In Guinea pigs, the observed increased masking of the AM CAP provides evidence for CIND after repeated noise exposure.