Auditory cortical neuron response differences under isoflurane versus pentobarbital anesthesia

Response properties of the middle layers of feline primary auditory cortex neurons to simple sounds were compared for isoflurane versus pentobarbital anesthesia in a within subject study control design. Initial microelectrode recordings were made under isoflurane anesthesia. After a several hour washout period, recordings were repeated at spatially matched locations in the same animal under pentobarbital. The median spatial separation between matched recording locations was 50 microns. Excitatory frequency tuning curves (n=71 pairs) to tone bursts and entrainment to click train sequences (n=64 pairs) ranging from 2 to 38 Hz were measured. Characteristic frequency and BW10 and BW30 were not different under either anesthetic. The spontaneous rate was slightly decreased (P<0.05) for isoflurane (median 4.2 spikes/s) compared to pentobarbital (median 5.8 spikes/s). Minimum median threshold and latency were elevated by 12 dB and 2 ms, respectively, under isoflurane. Entrainment to click sequences assumed a lowpass filter profile under both anesthetics, but was markedly impoverished under isoflurane. Responses to click sequences under isoflurane were phasic to the first click but had very poor following to subsequent elements. Compared to pentobarbital, isoflurane appears to have a profound impact on response sensitivity and temporal response properties of auditory cortical neurons.