Central nervous system changes associated with noise-induced hearing loss: an electron microscopic and freeze-fracture study of the chick nucleus magnocellularis.

This investigation assessed the effects of noise-induced hearing loss on the avian nucleus magnocellularis, the homologue of the mammalian cochlear nucleus. Fifteen-day-old chicks were exposed to high-intensity broad-band noise (132 dBA) for 24 to 72 hours. Hearing loss was documented with auditory brain-stem response recordings and scanning electron microscopy of the basilar papilla. Nucleus magnocellularis was assessed with thin-section electron microscopy and freeze-fracture techniques. Freeze-fracture preparations of noise-exposed animals showed a decrease in synaptic vesicle fusion sites at the synaptic active zone, a decrease in re-uptake of presynaptic membrane by coated vesicles, and a shift of the preferential fracture plane at the synapse from the presynaptic membrane to the post-synaptic membrane. These findings support the hypothesis that the effects of noise are not limited to the auditory periphery, but that transneuronal changes extend into the auditory central nervous system.