Regeneration of Auditory Nerve Following Complete Sectioning and Intrathecal Application of the IN-1 Antibody

Summary.

 The cochlear nerve of adult Lewis rats was following microsurgical exposure in the cerebellopontine angle (CPA). The lesions completely interrupted the auditory nerve axons at the lesion site producing ipsilateral deafness in all animals. The rats were then treated with a recombinant Fab fragment of the antibody IN-1 against nerve growth inhibitory proteins for one to two weeks. An age-matched control group of rats was treated with unspecific mouse IgG antibody. Because the cochlear nerve lesions resulted in significant neuronal apoptosis of spiral ganglion cells, neurotrophin-3 (NT-3) was applied to the lesion site immediately post-injury in some rats. Electrophysiological studies were carried out by recording the brainstem auditory evoked potentials (BAEP) before and immediately after the lesion, and at regular intervals up to 2 months after injury. Cochlear nerve fibres were anterogradely traced by horseradish peroxidase (HRP) or biotinylated dextran amine (BDA) injected into the spiral ganglion. The results achieved in this study were consistent with the following conclusions: 1) transection of the adult rat cochlear nerve at the CPA results in functional deafness, disappearance of BAEP, apoptosis of parent axotomized neurons of the spiral ganglion, and interruption of labelled axons close to the lesion site; 2) NT-3 is able to partially rescue axotomized neurons of the spiral ganglion; 3) injured cochlear nerve fibres show a limited spontaneous sprouting and regrowth response which does not lead to BAEP recovery; 4) intrathecal treatment with IN-1 directed against myelin-associated neurite growth inhibitory proteins promotes significant elongation of the injured fibres; and 5) the regenerating fibres seem to navigate to correct targets, and be able to establish synaptic connections for functional recovery as depicted by BAEP examinations.