Efferent-mediated protection of the cochlear base from acoustic overexposure by low doses of lithium

Many studies on anaesthetized animals and a few on awake animals have suggested that the cholinergic olivocochlear efferent feedback to outer hair cells can participate in the protection of the cochlea from acoustic overexposure. Lithium is known to stimulate acetylcholine synthesis and release in the brain and it is likely to act similarly at the level of the cochlear efferent synapses. We demonstrate here that, in the awake guinea-pig with a chronically implanted electrode on the round window of the cochlea, the temporary threshold shift induced by 1 minute exposure to different pure tones at around 90 dB sound pressure level (SPL) was reduced by as much as 40 dB, when exposure occurred after lithium treatment. The protection effect was not observed in anaesthetized animals. The effect was seen across the test frequency range of 6.4-12.5 kHz, suggesting that both 'fast' and 'slow' efferent effects are likely to be mediated by acetylcholine. Together our results provide new evidence that the olivocochlear efferents can provide a more efficient protection from acoustic overexposure when animals are awake.