Effects of isoflurane on auditory middle latency (MLRs) and steady-state responses (SSRs) recorded from the temporal cortex of the rat

Both MLRs and SSRs have been proposed for monitoring anesthesia since they are attenuated in a concentration-dependent manner by general anesthetics and since they provide reliable information on the level of consciousness. Our previous studies showed that MLR linear addition and other phenomena mainly related to the recovery cycle interact in a complex way to generate the SSRs recorded from the temporal cortex of the awake rat. This study was aimed at evaluating the behavior of MLRs and SSRs recorded from the rat temporal cortex in the awake state and under isoflurane anesthesia. Recordings were obtained by means of epidural electrodes which were put into a custom built air-tight box fitted with a high fidelity loudspeaker and openings for gas delivery and scavenging. Stimuli consisted of 0.1 ms condensation clicks presented at 95 dB peSPL with an inter-stimulus interval of 250 ms to obtain MLRs and as trains at 40, 50, 120, 240 and 480 Hz with an inter-train interval of 250 ms to obtain SSRs. Recordings were performed in the awake condition and under anesthesia with isoflurane at different concentrations (0.38, 0.76 and 1.1 vol% in oxygen as measured with an infrared analyzer). MLR waves showed a concentration-dependent latency increase under anesthesia. The amplitude was reduced with respect to mean awake baseline value within the 15 minutes of each concentration step (by 25% P1N1 and 35% N1P2 at 0.38 vol%; by 60% P1N1 and 56% N1P2 at 0.76 vol%) and the decrease was not dependent from isoflurane concentration. SSRs increased in phase and decreased in amplitude under anesthesia and the amplitude reduction was not predictable on the basis of MLR parameters. Our results point to a differential sensitivity of MLRs and SSRs to isoflurane and suggest that anesthesia could enhance the role of rate-dependent effects into SSR generation.