Cell death or survival: molecular and connectional conditions for olivocochlear neurons after axotomy

We aimed to determine whether rat olivocochlear neurons survive axotomy inflicted through cochlear ablation, or if they degenerate. To estimate their intrinsic potential for axonal regeneration, we investigated the expression of the transcription factor c-Jun and the growth-associated protein-43 (GAP43). Axonal tracing studies based on application of Fast Blue into the cochlea and calcitonin gene-related peptide immunostaining revealed that many, but not all, lateral olivocochlear neurons in the ipsilateral lateral superior olive degenerated upon cochleotomy. A decrease of their number was noticed 2 weeks after the lesion, and 2 months postoperative the population was reduced to approximately one quarter (27-29%) of its original size. No further reduction took place at longer survival times up to 1 year. Most or all shell neurons and medial olivocochlear neurons survived axotomy. Following cochleotomy, 56-60% of the lateral olivocochlear neurons in the ipsilateral lateral superior olive were found to co-express c-Jun and GAP43. Only a small number of shell and medial olivocochlear neurons up-regulated c-Jun expression, and only a small number of shell neurons expressed GAP43. Up-regulation of c-Jun and GAP43 in lateral olivocochlear neurons upon axotomy suggests that they have an intrinsic potential to regenerate after axotomy, but cell counts based on the markers Fast Blue and calcitonin gene-related peptide indicate that this potential cannot be exploited and degeneration is induced instead. The survival of one quarter of the axotomized lateral olivocochlear neurons and of all, or almost all, shell and medial olivocochlear neurons appeared to depend on connections of these cells to other regions than the cochlea by means of axon collaterals, which remained intact after cochleotomy.