Anti-inflammatory compounds improve spiral ganglion neuron survival after aminoglycoside-induced hair cell loss in rats

Spiral ganglion neurons (SGNs) relay auditory information from cochlear hair cells to the central nervous system. After hair cells are destroyed by aminoglycoside antibiotics, SGNs gradually die. However, the reasons for this cochlear neurodegeneration are unclear. We used microarray gene expression profiling to assess transcriptomic changes in the spiral ganglia of kanamycin-deafened and age-matched control rats and found that many of the genes upregulated after deafening are associated with immune/inflammatory responses. In support of this, we observed increased numbers of macrophages in the spiral ganglion of deafened rats. We also found, via CD68 immunoreactivity, an increase in activated macrophages after deafening. An increase in CD68-associated nuclei was observed by postnatal day 23, a time before significant SGN degeneration is observed. Finally, we show that the immunosuppressive drugs dexamethasone and ibuprofen, as well as the NAD salvage pathway activator P7C3, provide at least some neuroprotection post-deafening. Ibuprofen and dexamethasone also decreased the degree of macrophage activation. These results suggest that activated macrophages specifically, and perhaps a more general neuroinflammatory response, are actively contributing to SGN degeneration after hair cell loss.