Susceptibility of the sympathectomized ear to noise-induced hearing loss.

Unilaterally sympathectomiced rats were exposed to 100 dB Leq frequency-modulated noise for 1 month. Normotensive as well as 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. In addition, morphological analysis was carried out. It was found that 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 months old hypertensive rats, or in normotensive rats of 11 months. A slightly smaller loss was seen in the sympathectomized side in young normotensive rats. It was concluded that 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.