Auditory adaptation to sound intensity in conscious rats: 2-[F-18]-fluoro-2-deoxy-D-glucose PET study

Despite the importance of the adaptive process for discriminating the broad range of sound intensity, there have been few systemic investigations targeting the auditory mechanisms. In this study, the adaptation effect of sound intensity on the change in glucose metabolism in rat brains was examined using a PET technique. In the first experiment, broadband white noise sound (40, 60, 80, or 100 dB sound pressure level) was given for 30 min after an 2-[F-18]-fluoro-2-deoxy-D-glucose injection in an awake condition. In the second experiment, sound stimuli with an intensity modulation of 0, 0.5, and 5.0 Hz in frequency and at three intensity levels were used for examining the metabolism change according to the short time scale variation of the sound intensity. As a result, the metabolic activities in the bilateral cochlear nucleus, superior olivary complexes, and inferior colliculus were proportional to the sound intensity level, whereas the bilateral auditory cortical areas unexpectedly decreased as the sound intensity level increased in the first experiment. In the second experiment, the glucose metabolism in the auditory cortex was higher at 0.5 and 5.0 Hz modulation frequency than the 0.0 Hz modulation frequency, while retaining an inverse relationship with the sound intensity. The metabolism in inferior colliculus was higher at 5.0 Hz modulation frequency than 0.0 and 0.5 Hz modulation frequencies. Taken together, the auditory cortex metabolism seemed to be actively adapted to the average sound intensity, which indicates that it plays an important role in processing the broad range to sound intensity more than the other nucleus of the auditory pathway. Especially, this study demonstrated that the sound intensity-dependent glucose metabolism can be seen in a small rodent's brain stem level using 2-[F-18]-fluoro-2-deoxy-D-glucose PET functional neuroimaging.