Gas Anaesthesia Impairs Peripheral Auditory Sensitivity in Barn Owls (Tyto alba)

ABSTRACT

Auditory-nerve single-unit recordings were obtained from two groups of young barn owls, aged between posthatching day 11 and 86, in terminal experiments under two different anaesthetic regimes: ketamine (6-11 mg/kg) plus xylazine (around 2 mg/kg), or isoflurane (1-1.5%) in oxygen, delivered via artificial respiration. In a second series of minimally invasive experiments, auditory brainstem responses (ABR) were recorded in the same four adult barn owls (Tyto alba), aged between 5 and 32 months, under three different anaesthetic protocols: Ketamine (10 mg/kg) plus xylazine (3 mg/kg), isoflurane (1-1.5%) and sevoflurane (2-3%) in carbogen. Finally, the ABR measurements on adult owls were repeated in terminal experiments including more invasive procedures such as artificial respiration and higher isoflurane dosage. The main finding was a significant deterioration of auditory sensitivity in barn owls under gas anaesthesia, at the level of the auditory nerve, i.e., a very peripheral level of the auditory system. The effect was drastic in the young animals which suffered threshold elevations in auditory-nerve single-unit responses of 20 dB or more. ABR thresholds assessed repeatedly in experiments on adult owls were also significantly higher under isoflurane and sevoflurane, on average by 7 and 15 dB, compared to ketamine/xylazine. This difference already occurred with minimal dosages and was reversibly enlarged with increased isoflurane concentration. Finally, there was evidence for confounding detrimental effects associated with artificial respiration over many hours which suggested oxygen toxicity.

SIGNIFICANCE STATEMENT Anaesthesia and analgesia are necessary for most invasive experiments. Their effects are also a concern for studying normal neural and sensory functions. We show a significant deterioration of hearing sensitivity of the auditory nerve under gas anaesthesia (isoflurane or sevoflurane), compared to injection anaesthesia with ketamine/xylazine, in barn owls. This generalises similar findings across birds and mammals and suggests that while inhalants are widely recommended as safe and easy-to-use anaesthetics in veterinary contexts, they should only be used with great caution in auditory neurophysiology, even at the most peripheral level. Future important questions are whether the deterioration of sensitivity at the periphery generalises to other senses and what the precise mechanisms are that determine the species-specific extent of sensitivity loss.