Evidence of neuronal plasticity within the inferior colliculus after noise exposure: a study of evoked potentials in the rat

Recent investigations have implicated that the central nervous system has a role in the changes that occur in auditory function following acoustic trauma caused by noise exposure. These investigations indicate that the inferior colliculus may be the primary anatomical location in the ascending auditory pathway where noise-induced neuronal plasticity occurs, thereby resulting in changes in the neuronal processing of auditory information. In the present investigation, we show that the amplitudes of all peaks in the click-evoked response from the external nucleus of the inferior colliculus decrease during a 30 min exposure to a tone (104 dB sound pressure level (SPL) at 4 kHz and 8 kHz). After tone exposure, the amplitudes of two of the peaks of the response from the external nucleus of the inferior colliculus that reflect the input from more caudal structures slowly returned to baseline levels, whereas the amplitudes of the two peaks reflecting neuronal activity in the inferior colliculus increased above baseline levels and remained at the increased levels for at least 90 min following exposure to the tone.We also show that exposure to a 4 kHz tone at 104 dB SPL causes changes in the neuronal processing of tonebursts in the form of changes in the temporal integration function for one of the peaks of the response from the external nucleus of the inferior colliculus that originates in the inferior colliculus. Before tone exposure the amplitude of this peak decreased with increasing stimulus duration, but after tone exposure the amplitude of this peak was independent of the duration of the toneburst stimulus.We interpret these changes as evidence that noise exposure (tone exposure) causes changes in the excitability of the inferior colliculus that are not seen in more caudal structures, and these changes are probably a result of a change in the balance between inhibition and excitation in the inferior colliculus.