Influence of high-altitude hypoxic environments on the survival of cochlear hair cells and spiral ganglion neurons in rats

The aim of the present study was to observe the histological changes in the peripheral auditory system in rats at different time-points after relocating from low altitude to high altitude (3,600 m). The general physical condition of the rats was observed and cochlear tissue samples were obtained every month. The morphology and survival of the cochlear hair cells (HCs) were observed using cochlear surface preparation at 1, 30, 90, 120, 150 and 180 days after moving to the plateau area. Changes in spiral ganglion neurons (SGNs) were detected at different time-points using immunofluorescence technology on frozen sections. No obvious morphological changes were observed in the cochlear HCs within 1-3 months of the rats moving to the plateau area, and there was little loss of outer HCs (OHCs) at 3 months. Cell swelling, dislocation and loss of cochlear OHCs were apparent at 4 months, and the losses of cochlear OHCs and inner HCs (IHCs) were 54 and 39%, respectively at 6 months. The loss of SGNs was observed at 3 months, and there was a loss of 28-35% of SGNs during 3-6 months. Thus, a high-altitude hypoxic environment influenced the cochlear HCs in rats after moving to the plateau area in a time-dependent manner. The damage to SGNs occurred earlier than the HCs, although SGN damage was not aggravated with time. Furthermore, compared with cochlear HCs, cochlear SGNs were identified to be markedly more sensitive to hypoxia, and exerted an adaptive mechanism to protect neurons from hypoxia.