The temperature dependency of neural and hair cell responses evoked by high frequencies

A fine thermocouple, placed on the round window of the guinea pig cochlea, was used to measure temperature and electrical potentials. In acute experiments, rectal temperature was held constant at 38 degrees C while cochlear temperature was varied between 38 degrees C and 31.5 degrees C. Cochlear cooling to 36 degrees C caused elevated thresholds for the compound action potential (CAP) in response to tone bursts with frequencies above 24 kHz. CAP latencies increased for all frequencies tested (2 to 40 kHz). Cooling to lower temperatures produced larger latency increases and greater threshold shifts which extended to CAPs evoked by frequencies as low as 16 kHz. These CAP changes were fully reversible after a cochlear temperature of 38 degrees C was restored. In an attempt to create more uniform cooling of the cochlea, experiments were also conducted with round window thermo-couples chronically implanted in guinea pigs. After cooling the entire anesthetized animal to 36 degrees C, CAP thresholds were again elevated only for frequencies above 24 kHz. In other acute experiments, extracellular dc receptor potentials were recorded from the organ of Corti with micropipettes. Cooling caused a decreased sensitivity to tones at the characteristic frequency of the recording location (20 kHz) and had less effect on responses to lower frequencies. The receptor potential usually recovered after rewarming the cochlea. These data emphasize that temperature is an important parameter when electrophysiological measurements are being made. They also suggest an explanation for the reported discrepancy between behavioral and electrophysiological thresholds at high frequencies.