Electrical stimulation of the inferior colliculus at low rates protects the cochlea from auditory desensitization

The effects of inferior collicular (IC) stimulation on cochlear responses were tested with pulsed electrical trains and with 1 min long continuous bursts. Pulsed trains did not cause any effects at the contralateral cochlea. However, a 1 min burst, containing pulses at low rates, was able to significantly reduce temporary threshold shifts (TTS) in cochlear sensitivity caused by a loud sound exposure. Intracochlear perfusion of hexamethonium blocked this effect. The time course of the hexamethonium blocking action paralleled its blocking action on the cochlear effects of electrical stimulation at the brainstem of an auditory afferent pathway, the crossed olivocochlear bundle (COCB). The protective IC effects were persistent and TTS reductions could be obtained even with a 5 min delay between IC stimulus and the loud sound. However, these persistent protective effects did not appear to occur at the cochlea. Finally, electrical stimulation at the IC ipsilateral to a cochlea exposed to loud sound also reduced TTS, but only by smaller amounts and at higher stimulation rates. Thus the IC appears to provide a strong descending influence that modulates the excitability levels of the olivocochlear nuclei in the brainstem. Both crossed and uncrossed OCB appear to be involved and able to reduce TTS. It is proposed that the protective effects may be due solely to the medial olivocochlear system and possibly only those fibres originating from one of the nuclei of the medial system.