Susceptibility of the sympathectomized ear to noise-induced hearing loss

Abstract

Unilaterally sympathectomized rats were exposed to 100 dB Leq [energy-equivalent sound level] FM noise for 1 mo. Normotensive and spontaneously hypertensive animals (with a blood pressure of above 200 mmHg) were investigated. Auditory sensitivity was determined by auditory brainstem responses to 1/3-octave filtered sine waves in the frequency range 0.8-20.0 kHz. A morphological analysis also was carried out. The sympathetic innervation to the inner ear of the rat originated in, or passed through the ipsilateral superior cervical ganglion. Sympathectomy did not alter pre-exposure hearing thresholds nor influence the size of the noise-induced hearing loss either in 3 or 11 mo. old hypertensive rats, or in normotensive rats of 11 mo. A slightly smaller loss was seen in the sympathectomized side in young normotensive rats. The sympathetic does not exert a protection of the inner ear against functional disturbances in hypertension, neither during basal metabolic condition nor during extreme conditions, i.e., during noxious noise exposure.