Alteration of susceptibility to noise-induced hearing loss through interruption of sympathetic inputs to the cochlea
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In 1966, sympathetic fibers were identified in the cochlea, terminating on the cochlear artery and near the auditory nerve fibers at the level of the habenula perforata (Spoendlin and Lichtensteiger, 1966). The cochlear sympathetic fibers originate in the stellate ganglion (SG) and the superior cervical ganglion (SCG). Both sets of fibers have a demonstrated effect on cochlear blood flow (CBF) (Laurikainen et al., 1993, 1994, 1997; Ren et al., 1993). The SCG fibers have been implicated in modulation of susceptibility to noise-induced hearing loss (NIHL) (Borg, 1982; Hildesheimer et al., 1991; Horner et al., 2001; Hildesheimer et al., 2002). Ablation of the SCG generally leads to protection from NIHL, but the previous studies do not separate the effects of SCG ablation on temporary and permanent threshold shift (TTS and PTS), and there is also some question about whether the effect is unilateral or bilateral. In the current study, we explore the effects of unilateral and bilateral SCG ablation on threshold shift from noise exposure, as measured with inferior colliculus (IC) evoked potentials, distortion product otoacoustic emissions (DPOAE), and outer hair cell (OHC) loss. SCGs were isolated at the level of the bifurcation of the carotid artery and removed unilaterally in 15 chinchillas. Another 8 chinchillas underwent bilateral ablation. 20 animals were employed as sham controls. Each animal was implanted with IC electrodes and exposed to an octave band noise centered at 4 kHz for one hour at 110 dB SPL. Hearing thresholds and DPOAE input/output functions were measured at six hours, one day, three days, one week, and three weeks after the noise. After the three-week test, animals were sacrificed and OHCs were counted for cochleograms. Results showed improved recovery of DPOAE amplitudes after noise exposure in ears that underwent SCGectomy, as well as lower evoked potential threshold shifts relative to sham controls. Effects of SCGectomy on OHC loss were small. Results of the study suggest that sympathetic fibers do exert some influence on susceptibility to noise, but the influence may not be a major one.