Contributions of age, cochlear integrity, and auditory environment to avian cochlear nucleus metabolism.

Most commercially raised broiler chickens display progressive cochlear degeneration with age [Hear. Res. 166 (2002) 82]. Recent work examining the effects of age and cochlear degeneration on avian cochlear nucleus (nucleus magnocellularis, NM) metabolism showed that changes in metabolic activity occur with age and cochlear damage [Hear. Res. 175 (2003) 101]. The auditory environment also differed between facilities housing young and adult birds. The relative contributions of age, cochlear degeneration, and auditory environment to these changes in NM metabolism are unknown. Using cytochrome oxidase (CO) histochemistry, NM neuron metabolism is examined in several age groups of birds under varying conditions. When normal cochlear integrity and auditory environment are held constant, CO staining is significantly decreased in adult vs. young birds. When age and auditory environment are held constant, CO staining is significantly decreased in birds with damaged vs. normal cochleae. When age and normal cochlear integrity are held constant, CO staining is significantly decreased in birds living in a quiet vs. noisy environment. All factors examined cause changes in CO staining, which is indicative of NM metabolic activity. Results are discussed in the context of mitochondrial aging, afferent regulation, and auditory deprivation and enrichment.