Isoflurane/N₂O anesthesia suppresses narrowband but not wideband inhibition in dorsal cochlear nucleus

Anesthesia alters the response properties of neurons in the dorsal cochlear nucleus (DCN). Barbiturates decrease spontaneous activity and the prevalence of inhibitory responses, so that DCN principal cells show less inhibition by narrowband stimuli (e.g. tones at best frequency). Here we present the effects on cat DCN of anesthesia using isoflurane plus nitrous oxide (N₂O). Because the cellular anesthetic mechanisms of isoflurane differ from those of pentobarbital, the effects of the two anesthetics in DCN might be different. The strength of two inhibitory circuits in the DCN, the narrowband and wideband inhibitor, were studied and compared with results in unanesthetized decerebrate animals. The primary effects of isoflurane/N₂O anesthesia were to lower spontaneous activity and increase the thresholds of units. All the response types seen in the decerebrate preparation were also seen with isoflurane/N₂O, but the prevalence of predominantly inhibitory responses to narrowband stimuli (type IV units) decreased (from ∼31% to ∼11%). However, responses to band-reject noise were similar to those seen in unanesthetized animals. Together, these results suggest that the effects of isoflurane/N₂O are primarily on the narrowband inhibitory circuit, rather than the wideband inhibitor.