Morphology of the cochlear nucleus in CBA/J mice with chronic, severe sensorineural cochlear pathology induced during adulthood.

The effects of chronic cochlear impairment on morphological features of the adult cochlear nucleus (CN) were assessed in CBA/J mice in which severe sensorineural damage had been induced by exposure to intense noise. Sections from various CN subdivisions, stained for Nissl substance and fibers, were quantitatively evaluated in four groups of noise-exposed mice that differed with regard to the age at noise exposure (2, 6, or 11 months), age at the time the CN was evaluated (6, 11, or 24 months), and the duration (chronicity) of sensorineural impairment (4, 5, 13, or 18 months). Like-aged, non-exposed CBA mice were used as controls, so the effects of peripheral damage and aging could be compared. Cochlear damage produced significant changes in CN subdivisions thought to receive the heaviest input from cochlear afferents (anteroventral CN, octopus cell area, dorsal CN layer III). These changes included a reduction of neuropil volume, reductions in neuron size, and increases in neuronal packing density that were complementary to reduced volume in these subdivisions. Effects on neuron number were minimal in all subdivisions. Central changes in noise-exposed mice were absent or diminished in DCN layers I and II, which receive relatively less input from primary fibers. The age at onset and chronicity of damage had little to do with the severity of central effects of cochlear damage. The effects of cochlear damage were not additive with age-related changes seen in the old controls.