Effects of Gentamicin on Guinea Pig Vestibular Ganglion Function and on Substance P and Neuropeptide Y

Previous studies have demonstrated that following intratympanic gentamicin application in the guinea pigs, vestibular evoked myogenic potentials VEMPs were absent regardless of stimulation mode using either air-conducted sound ACS stimuli or galvanic vestibular stimulation GVS. Ultrastructurally, both type I hair cells and their calyx terminals were distorted in the saccular macula. However, little is known about the toxic effects of gentamicin on the vestibular ganglion VG. In this study, absent ACS-and GVS- VEMPs were noted in all the gentamicin-treated ears 100%, which were confirmed by the substantial loss of sensory hair cells in the saccular macula. Moreover, dramatic up- regulation of growth associated protein-43 GAP-43 expression was detected in the ipsilateral VG neurons. The mean percentage of substance P-like immunoreactive SP-LI neurons in the treated VG 81.8 +/- 1.9% was significantly higher than that in the control VG 68.6 +/- 3.3%. Conversely, the mean percentage of neuropeptide Y-like immunoreactive NPY-LI neurons in the treated VG 13.7 + /- 3.8% was dramatically lower than that in the control VG 49.0 +/- 3.8%. Double labeling results shown 82% of SP-LI and 16% of NPY-LI neurons coexpressed with GAP-43, suggested that SP accumulating coincided with NPY decreasing in regenerating VG neurons after gentamicin treatment. Overall , the changes in SP and NPY expression in VG neurons after gentamicin treatment were like to those in the superior cervical ganglion following sympathectomy