Subhypnotic doses of isoflurane impair auditory discrimination in rats

Isoflurane at subhypnotic doses is known to affect cellular and network activity in the auditory pathway, but the behavioral effects of these concentrations of isoflurane on auditory processing have not been tested previously. The authors tested the hypothesis that subhypnotic doses of isoflurane would impair auditory discrimination in rats.

Rats were tested on their ability to discriminate up versus down frequency-modulated sweeps using three different pairs of sweeps ("Long," "Med," "Short"), whose frequency range and duration were varied systematically to make the discrimination more difficult. Rats were tested daily in the absence and presence of isoflurane at 0.2% or 0.4%. The effects of isoflurane (0%, 0.2%, and 0.4%) on performance (= % correct) and efficiency (= time/trial) were assessed using regression analysis.

The effect of isoflurane was stimulus-dependent: performance for the Long stimulus pair was unaffected by isoflurane, performance on the Med stimulus pair was impaired only by 0.4% isoflurane, and performance on the Short stimulus pair was impaired by both 0.2% and 0.4% isoflurane. In contrast, isoflurane decreased efficiency equally for all stimulus pairs at 0.4% and had no effect at 0.2%.

The stimulus dependence of the effect of isoflurane on performance suggests that it is unlikely this effect was secondary to effects on memory, motivation, or motor function. These data indicate that doses of isoflurane known to produce modest effects on neural responses alter cortical sensory processing.