Microglial activation in the cochlear nucleus after early hearing loss in rats

Microglia are highly specialized tissue macrophages in the central nervous system. Their activation in the auditory system has been reported in adult hearing loss models, but their status in the developing auditory system is less understood. Therefore, we investigated microglial status in the cochlear nucleus (CN) during normal developing periods and after exposing rats to amikacin, a potent ototoxin, around the time of hearing onset.

To develop the deafness model, rats were administered with a daily intraperitoneal injection of amikacin (500 mg/kg) from postnatal day 7 (P7) to P15. To evaluate the expression of ionized calcium binding adaptor molecule 1 (Iba1), we performed immunohistochemical analysis using rat brains from P10-60. To compare the expression of microglia-related gene, reverse transcription quantitative polymerase chain reaction (RT-qPCR) analysis were performed.

Immunohistochemical analysis revealed that, under normal conditions, microglia had relatively large cell bodies with several extended processes that surrounded other cells at P10, while the sizes and number of these cells gradually decreased afterward. In contrast, when amikacin was administered from P7 to P15, microglia maintained large cell bodies with relatively shorter processes at both P15 and P21. Furthermore, RT-qPCR analysis revealed upregulation of genes including phagocytotic and anti-inflammatory markers after amikacin administration.

These results suggest that microglia are activated in the CN, and they may contribute to tissue remodeling after early hearing loss in the developing auditory system.