PLASTICITY OF THE AUDITORY BRAIN-STEM - EFFECTS OF COCHLEAR ABLATION ON GAP-43 IMMUNOREACTIVITY IN THE RAT

In the adult brain, expression of the growth associated protein GAP-43 may serve as an indicator of synaptic remodeling. We have studied loc alization and time course of the re-expression of GAP-43 following dea fening through cochlear ablation. As a consequence of unilateral cochl ear lesioning. a substantial increase in the expression of GAP-43 was observed in the neuropil of all subnuclei of the ipsilateral cochlear nuclear complex. This expression of GAP-43 occurred in well-defined fi bers and boutons. In the ventral cochlear nuclei, boutons immunoreacti ve for GAP-43 were often localized on cell bodies. However, they were found only on selected subpopulations of cochlear nucleus neurons, i.e ., on cell bodies containing glutamate or calretinin immunoreactivity, but apparently not on GABAergic neurons. Olivocochlear neurons must h ave been axotomized by the operation. Following cochlear ablation, a d ramatic re-expression of GAP-43 occurred in cell bodies of the ipsilat eral lateral superior olive but not in the ventral nucleus of the trap ezoid body. Position and number of these cells suggested that most, if not all, of them serve the lateral olivocochlear bundle. However, alt hough axon collaterals are given off by certain types of olivocochlear neurons, a direct involvement of the immunoreactive cell bodies in th e emergence of GAP-43 In the cochlear nucleus is not obvious. A transi ent rise of GAP-43 immunoreactivity that could not be attributed to ax otomized neurons was observed in the contralateral dorsal cochlear nuc leus and in the ipsilateral inferior colliculus. Given the functional significance attributed to GAP-43, we conclude that the sudden loss of spiral ganglion cells leads to a reactive synaptogenesis in complex p atterns across several auditory brainstem nuclei. (C) 1997 Wiley-Liss, Inc.