Cytochrome oxidase response to cochlea removal in chicken auditory brainstem neurons

Changes in cytochrome oxidase (CO) activity were studied in the chick brainstem auditory nuclei, n. magnocellularis (NM) and n. laminaris (NL), following unilateral cochlea removal. Chickens aged 10 days or 56 weeks underwent unilateral cochlea removal. Following survival periods of 30 minutes to 14 days for the 10-day-old birds and 6 hours or 14 days for the 56-week-old birds, the animals were perfused with paraformaldehyde/glutaraldehyde fixative. Cryostat sections of the brainstem were then prepared for CO histochemistry. Microdensitometry was used to quantify the difference in CO staining in NM and NL ipsilateral and contralateral to the cochlea removal. Since the cochlea projects to the ipsilateral NM, the contralateral NM was used as a within-animal control. In normal chickens, NM cell bodies and the cell bodies and dendrites of NL neurons stain darkly for CO in both young and adult birds. In 10-day-old birds, there is no significant change in CO staining in NM from 30 minutes to 3 hours after cochlea removal. Then, a rapid biphasic change in CO staining was found in the ipsilateral NM. An increase in staining was observed 6 to 24 hours postoperatively, followed by a decrease in CO staining at 3- to 14-day survival times. In the 56-week-old birds, no increases in CO staining were observed 6 hours after cochlea removal, but a decrease in CO staining was found 14 days postoperatively. In NL, no changes were observed until 3 days (10-day-old birds) or 14 days (56-week-old birds) after cochlea removal. Then a decrease in CO staining was observed in the dendritic and glial/fiber regions of NL containing axons from the deafferented NM. Thus it appears that afferent input has a regulatory effect on the oxidative metabolism of neurons in the chicken auditory brainstem nuclei, an effect that differs with the age of the animal at the time of afferent manipulation.