Auditory brainstem of the ferret: Long survival following cochlear removal progressively changes projections from the cochlear nucleus to the inferior colliculus

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Abstract
Some effects on auditory brainstem connections of long (1–2.3 years) survival following unilateral cochlear removal in infant and adolescent ferrets were examined by making multiple injections of wheat germ agglutinin conjugated to horseradish peroxidase (WGA‐HRP) in either the left or the right inferior colliculus (IC). Previous studies have shown that, in normal adult ferrets, about 50 times as many cochlear nucleus (CN) neurons project to the contralateral as to the ipsilateral IC. Right cochlear removal at P25 increased, within 30 days, the number of retrogradely labeled left CN neurons projecting to the left ipsilateral IC by 17% (from n = 235 to n = 275), relative to normals. In this study, longer survival (3 months to 1 year) after right cochlear removal at P25 resulted in larger increases (38–47%; n ≈ 100) in the number of neurons labeled in the left CN after injections of WGA‐HRP in the left IC. No change occurred in the number of neurons labeled in the right CN. Taken together, the results of these experiments show that the ratio of the number of labeled neurons in the left CN to that in the right CN increases progressively with survival time out to the maximum time tested (1 year). In contrast to these results, we have previously reported that right cochlear removal at P90 did not change the number of neurons projecting from the left CN to the left IC after 90 days of survival. However, in this study, very long survival (2–3 years) following right cochlear removal at P90 resulted in an increased (51%, from n = 235 to n = 355) number of left CN neurons labeled by WGA‐HRP injections into the left IC, relative to normals. The increased number of labeled neurons included neurons throughout each division of the CN and all of the principal morphological types. In a separate series of experiments involving long survival (1‐2 years), right cochlear removal at P25 or P40 did not significantly change the number of neurons in either CN retrogradely labeled by injections of WGA‐HRP in the right IC, or the ratio between the number of neurons labeled in each CN. Long survival following cochlear removal at P25–P90 did not result in any loss of neurons in the ipsilateral CN or in any shrinkage of CN neurons further than the 10–20% seen at a shorter survival time (90 days). These results show that at least one pathway within the auditory brainstem continues to be changed in later life following either early or late cochlear removal. They provide a possible anatomical basis for long‐term changes seen in sensory systems following lesions of the sense organs of mature animals.

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