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In this study, we used image analysis to assess changes in calretinin immunoreactivity in the lateral (LSO) and medial (MSO) superior olivary nuclei in ferrets 2 months after unilateral cochlear ablations at 30–40 days of age, soon after hearing onset. These two nuclei are the first significant sites of binaural convergence in the ascending auditory system, and both receive direct projections from the deafferented cochlear nucleus. Cochlear ablation results in a decrease in the overall level of calretinin immunostaining within the LSO ipsilaterally compared with the contralateral side and with control animals and within the MSO bilaterally compared with control ferrets. In addition, the level of calretinin immunostaining ipsilaterally within neurons in the LSO was significantly less in cochlear ablated than control animals. In contrast, there was no effect of cochlear ablation on the level of calretinin immunostaining within neurons either in the contralateral LSO or in the MSO. These results are consistent with a downregulation in calretinin within the neuropil of MSO bilaterally and LSO ipsilaterally, as well as a downregulation in calretinin within somata in the ipsilateral LSO as a result of unilateral cochlear ablation soon after hearing onset. Thus, cochlear-driven activity appears to affect calcium binding protein levels in both neuropil and neurons within the superior olivary complex. J. Comp. Neurol. 470:63–79, 2004. © 2004 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]

In many systems, including ascending auditory pathways, calcium-binding proteins are markers of specific neuronal circuits. Previous studies suggest that calretinin immunostaining may be a specific marker for circuits in the inferior colliculus (IC) that code timing information. We undertook experiments to determine the changes in calretinin immunostaining in the IC that take place in response to cochlear ablation. Cochlear ablation was performed unilaterally in ferrets just after hearing onset. Animals survived for 2–3 months after ablation and brains were then processed for calretinin immunocytochemistry. The mean optical density and stained area of the calretinin immunopositive plexus in the IC were determined for five coronal sections through the right and left IC. In controls (n = 3), measurements of these parameters in the central nucleus of the IC showed symmetry between the two sides. In experimental animals (n = 8) the calretinin immunopositive plexus contralateral to the cochlear ablation was denser and larger than that in either the ipsilateral IC or in the IC of control animals. The calretinin plexus in the ipsilateral IC was slightly less dense and smaller than in controls but the differences did not reach statistical significance. IC volume measurements and synaptophysin immunostaining analysis in the central nucleus of the IC revealed no statistical differences between ablated and control animals or between the two sides in ablated animals. The significant increase in both mean optical density and immunostained area of the calretinin plexus in the IC contralateral to the cochlear ablation may reflect an upregulation in calretinin expression in the nuclei that contribute to this plexus. J. Comp. Neurol. 460:585–596, 2003. © 2003 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]