Evidence for a microglial reaction within the vestibular and cochlear nuclei following inner ear lesion in the rat

Following unilateral inner ear lesion, astrocytes undergo hypertrophy in the deafferented vestibular and cochlear nuclei as shown by an increase in the level of glial fibrillary acid. The present study extends our understanding of vestibular and cochlear system plasticity by examining microglial changes in these deafferented nuclei. The microglial reaction was studied 1, 2, 4, 8, 14, 21, 28 and 42 days following the lesion with a monoclonal OX-42 antibody and lectins (Griffonia simplicifolia, B4 isolectin) labelled with horseradish peroxidase or fluorescein. The deafferented nuclei were also examined for apoptotic cells by terminal transferase-mediated nick end labelling of nuclear DNA fragments. In control and sham-operated rats, the distribution of the resting microglial cells was uniform in both the vestibular and cochlear nuclei. In the deafferented vestibular complex, the microglial cells increased in number, became hypertrophied and were distributed in the medial, lateral, superior and inferior vestibular nuclei. Reactive microglial cells were also detected in the ipsilateral cochlear nuclei. Some of the immunostained cells were hypertrophic whereas others presented an ameboid morphology with few short and stout processes. The microglial reaction was confined to the antero- and posteroventral cochlear nuclei. Finally, reactive microglia was also observed in the prepositus hypoglossi ipsilateral to the lesion. The microglial reactions within the prepositus hypoglossi, the vestibular and the cochlear nuclei were detectable as early as one day after the lesion and persisted several weeks in both the vestibular and cochlear nuclei. Apoptotic cells were not detected in the vestibular nuclei at any stage following the lesion. In contrast, terminal deoxynucleotidyl transferase-mediated digoxygenin-11-dUTP nick end labelling-positive cells were first detected in the deafferented cochlear nuclei on the 3rd day following the lesion. They reached an apparent maximum by day 8 and then declined until day 24. Double labelling experiments demonstrate that these cochlear terminal deoxynucleotidyl transferase-mediated digoxygenin-11-dUTP nick end labelling-positive cells were also lectin-positive suggesting that reactive cochlear lectin-positive microglia cells were eliminated by a programmed cell death. Our results establish the two experimental models as reliable tools to understand the role of microglia in adult brain plasticity. The cochlear microglial reaction was probably induced by the degeneration of the acoustic nerve which follows the acoustic ganglion destruction. Interestingly, the same reasoning cannot apply to the vestibular microglial reaction following unilateral labyrinthectomy: the vestibular ganglion was spared and the primary vestibular neurons did not degenerate, at least during the first week following the lesion.